[llvm] [llvm] Disable some LLVM arguments in runtimes mode (PR #73031)

[Joseph Huber via llvm-commits]

================
@@ -151,6 +151,9 @@ endif()
 # Avoid checking whether the compiler is working.
 set(LLVM_COMPILER_CHECKED ON)
 
+# This can be used to detect whether we're in the runtimes build.
+set(LLVM_RUNTIMES_BUILD ON)
+
 # Handle common options used by all runtimes.
 include(AddLLVM)
 include(HandleLLVMOptions)
----------------
jhuber6 wrote:

This change was initially to unblock me on some issues with GPU compilation, but I'm happy to expand the scope if others are in agreement.

There's a handful of clang options we may want to keep to remove some regressions? Here's the set of flags that the `libc` project has right now:
```
-fPIC                                                                                                                                                                                                                   
-fno-semantic-interposition                                                                                                                                                                                             
-fvisibility-inlines-hidden                                                                                                                                                                                             
-Werror=date-time                                                                                                                                                                                                              
-Werror=unguarded-availability-new                                                                                                                                                                                             
-Wall -Wextra                                                                                                                                                                                                                  
-Wno-unused-parameter                                                                                                                                                                                                          
-Wwrite-strings -Wcast-qual                                                                                                                                                                                                    
-Wmissing-field-initializers                                                                                                                                                                                                   
-Wimplicit-fallthrough                                                                                                                                                                                                         
-Wcovered-switch-default                                                                                                                                                                                                       
-Wno-noexcept-type                                                                                                                                                                                                             
-Wnon-virtual-dtor                                                                                                                                                                                                             
-Wdelete-non-virtual-dtor                                                                                                                                                                                                      
-Wsuggest-override                                                                                                                                                                                                             
-Wstring-conversion                                                                                                                                                                                                            
-Wmisleading-indentation                                                                                                                                                                                                       
-Wctad-maybe-unsupported                                                                                                                                                                                                       
-fdiagnostics-color                                                                                                                                                                                                            
-ffunction-sections                                                                                                                                                                                                            
-fdata-sections
```

Do any of these look integral? We probably want to keep `-fPIC -ffunction-sections -fdata-sections` if nothing else. Warnings should probably also be somewhat consistent.

https://github.com/llvm/llvm-project/pull/73031