[CMake] Finish off the clang-cl self host build

[zturner]

Summary

This allows clang-cl to self-host cleanly with no magic setup
steps required.

After this patch, all you have to do is set CC=CXX=clang-cl and
run cmake -G Ninja.

These changes only exist to support C++ features which are
unsupported in clang-cl, so regardless of whether the user
specifies they want to use them, we still have to disable them.

[rnk]

cmake/modules/AddLLVM.cmake

13

Won't 'NOT LLVM_EH_UNSUPPORTED' always be true? Normally it's undefined, but you set it to 0 in HandleLLVMOptions.

23

This should be under if(CLANG_CL) right? Otherwise gtest binaries can't recover from crashes and run the rest of the tests, which is semi-useful functionality.

cmake/modules/HandleLLVMOptions.cmake

31–32

I'm starting to feel like clang-cl should just silently ignore /Fd. Mind sending that patch instead? Otherwise, this needs a comment about /Fd.

[zturner]

Updated with suggestions. Also removed the unused arguments warning. I will make clang-cl ignore that argument silently in a separate patch.

[rnk]

lgtm

woot!`

cmake/modules/AddLLVM.cmake

25

Can you put something like # FIXME: Remove this when LLVM's SEH support is finished.?

cmake/modules/AddLLVM.cmake

 
   # LLVM_REQUIRES_EH is an internal flag that individual
   # targets can use to force EH
-  if(LLVM_REQUIRES_EH OR LLVM_ENABLE_EH)
+  if((LLVM_REQUIRES_EH OR LLVM_ENABLE_EH) AND NOT LLVM_EH_UNSUPPORTED)
     if(NOT (LLVM_REQUIRES_RTTI OR LLVM_ENABLE_RTTI))
       message(AUTHOR_WARNING "Exception handling requires RTTI. Enabling RTTI for ${name}")
       set(LLVM_REQUIRES_RTTI ON)

       list(APPEND LLVM_COMPILE_FLAGS "-fno-exceptions")
     elseif(MSVC)
       list(APPEND LLVM_COMPILE_DEFINITIONS _HAS_EXCEPTIONS=0)
+      list(APPEND LLVM_COMPILE_DEFINITIONS "GTEST_HAS_SEH=0")
       list(APPEND LLVM_COMPILE_FLAGS "/EHs-c-")
     endif()
   endif()

cmake/modules/HandleLLVMOptions.cmake

       if (CMAKE_CXX_SIMULATE_VERSION VERSION_LESS 18.0)
         message(FATAL_ERROR "Host Clang must have at least -fms-compatibility-version=18.0")
       endif()
+
+      set(CLANG_CL 1)
+      set(LLVM_EH_UNSUPPORTED 0)
+      set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Qunused-arguments")
+      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Qunused-arguments")
+
     elseif(NOT LLVM_ENABLE_LIBCXX)
       # Otherwise, test that we aren't using too old of a version of libstdc++
       # with the Clang compiler. This is tricky as there is no real way to