[clang] [flang] [llvm] [mlir] [polly] [CMake][LLVM] Add PCH infrastructure and LLVMSupport PCH (PR #176420)

[Maksim Levental via cfe-commits]

================
@@ -5,6 +5,8 @@ set(LLVM_LINK_COMPONENTS
 add_clang_executable(c-index-test
   c-index-test.c
   core_main.cpp
+
+  DISABLE_PCH_REUSE # Prevent CMake error with C source files.
----------------
makslevental wrote:

this is a really stupid nit i'm sorry but can you move these comments to just above the option, like 

```
 # Prevent CMake error with C source files.
DISABLE_PCH_REUSE
```

reason being that formatters can't move these things around because the comment is treated as part of the line.

https://github.com/llvm/llvm-project/pull/176420