[CMake] Don't use -Bsymbolic-functions for MinGW targets

[mstorsjo]

Summary

This is an ELF specific option which isn't supported for Windows/MinGW
targets, even if the MinGW linker otherwise uses a ld.bfd like linker
interface.

In llvm-shlib, there's already a separate if statement for

if (NOT LLVM_LINKER_IS_SOLARISLD AND NOT MINGW)

but I don't know if the solaris linker supports -Bsymbolic-functions
or not.

[MaskRay]

but I don't know if the solaris linker supports -Bsymbolic-functions or not.

According to a Solaris "man pages section 1: User Commands", -Bsymbolic-functions is supported.

clang/tools/clang-shlib/CMakeLists.txt

add_clang_library(clang-cpp
                   SHARED
                   ${INSTALL_WITH_TOOLCHAIN}
                   clang-shlib.cpp
                   ${_OBJECTS}
                   LINK_LIBS
                   ${_DEPS})
 # Optimize function calls for default visibility definitions to avoid PLT and
 # reduce dynamic relocations.
-if (NOT APPLE)
+if (NOT APPLE AND NOT MINGW)
   target_link_options(clang-cpp PRIVATE LINKER:-Bsymbolic-functions)
 endif()

llvm/tools/llvm-shlib/CMakeLists.txt

if(("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux") OR (MINGW) OR (HAIKU)
     ${LLVM_LIBRARY_DIR}/tools/llvm-shlib/simple_version_script.map)
 
     # GNU ld doesn't resolve symbols in the version script.
     set(LIB_NAMES -Wl,--whole-archive ${LIB_NAMES} -Wl,--no-whole-archive)
     if (NOT LLVM_LINKER_IS_SOLARISLD AND NOT MINGW)
       # Solaris ld does not accept global: *; so there is no way to version *all* global symbols
       set(LIB_NAMES -Wl,--version-script,${LLVM_LIBRARY_DIR}/tools/llvm-shlib/simple_version_script.map ${LIB_NAMES})
     endif()
+    if (NOT MINGW)
       # Optimize function calls for default visibility definitions to avoid PLT and
       # reduce dynamic relocations.
       # Note: for -fno-pic default, the address of a function may be different from
       # inside and outside libLLVM.so.
       target_link_options(LLVM PRIVATE LINKER:-Bsymbolic-functions)
+    endif()
   elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin")
     set(LIB_NAMES -Wl,-all_load ${LIB_NAMES})
   endif()
 
   target_link_libraries(LLVM PRIVATE ${LIB_NAMES})
 
   if (APPLE)
     set_property(TARGET LLVM APPEND_STRING PROPERTY