[llvm] Initial changes for llvm shared library build using explicit visibility annotations (PR #96630)

[Petr Hosek via llvm-commits]

================
@@ -839,20 +841,36 @@ endif()
 
 if(MSVC)
   option(LLVM_BUILD_LLVM_C_DYLIB "Build LLVM-C.dll (Windows only)" ON)
-  # Set this variable to OFF here so it can't be set with a command-line
-  # argument.
-  set (LLVM_LINK_LLVM_DYLIB OFF)
   if (BUILD_SHARED_LIBS)
     message(FATAL_ERROR "BUILD_SHARED_LIBS options is not supported on Windows.")
   endif()
-else()
-  option(LLVM_LINK_LLVM_DYLIB "Link tools against the libllvm dynamic library" OFF)
+ else()
   option(LLVM_BUILD_LLVM_C_DYLIB "Build libllvm-c re-export library (Darwin only)" OFF)
-  set(LLVM_BUILD_LLVM_DYLIB_default OFF)
-  if(LLVM_LINK_LLVM_DYLIB OR LLVM_BUILD_LLVM_C_DYLIB)
-    set(LLVM_BUILD_LLVM_DYLIB_default ON)
-  endif()
-  option(LLVM_BUILD_LLVM_DYLIB "Build libllvm dynamic library" ${LLVM_BUILD_LLVM_DYLIB_default})
+endif()
+
+set(CAN_BUILD_LLVM_DYLIB OFF)
+if(NOT MSVC OR LLVM_BUILD_LLVM_DYLIB_VIS)
----------------
petrhosek wrote:

You could consider making it an internal variable:
```
set(LLVM_BUILD_LLVM_DYLIB_VIS OFF CACHE INTERNAL "")
```

https://github.com/llvm/llvm-project/pull/96630