Add CMake option for using /DEBUG:GHASH

[zturner]

Summary

This way when self-hosting we can just say -DLLVM_ENABLE_DEBUG_GHASH=ON without having to mess around manually with cxxflags and ldflags.

[rnk]

lgtm

[aganea]

Is this still to be commited?

[rnk]

Let's land this, I want to try it out. :)

[rnk]

We are moving to make ghash the default, so I don't think we need this. I have a follow-up to pass -gcodeview-ghash: D102888 I think we can close this as obsolete.

llvm/cmake/modules/HandleLLVMOptions.cmake

if( MSVC )
   if (LLVM_ENABLE_PDB AND uppercase_CMAKE_BUILD_TYPE STREQUAL "RELEASE")
     append("/Zi" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
     # /DEBUG disables linker GC and ICF, but we want those in Release mode.
     append("/DEBUG /OPT:REF /OPT:ICF"
           CMAKE_EXE_LINKER_FLAGS CMAKE_MODULE_LINKER_FLAGS
           CMAKE_SHARED_LINKER_FLAGS)
   endif()
 
+  OPTION(LLVM_ENABLE_DEBUG_GHASH OFF)
+  if (LLVM_ENABLE_DEBUG_GHASH)
+    if (CLANG_CL AND LINKER_IS_LLD_LINK)
+      append("-mllvm -emit-codeview-ghash-section" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+      append("/DEBUG:GHASH" CMAKE_EXE_LINKER_FLAGS CMAKE_MODULE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
+    else()
+      if (NOT CLANG_CL)
+        message(WARNING "Option LLVM_ENABLE_DEBUG_GHASH is incompatible with cl.exe, clang-cl is required.")
+      endif()
+      if (NOT LINKER_IS_LLD_LINK)
+        message(WARNING "Option LLVM_ENABLE_DEBUG_GHASH is incompatible with link.exe, lld-link is required.")
+      endif()
+    endif()
+  endif()
+
   # /Zc:strictStrings is incompatible with VS12's (Visual Studio 2013's)
   # debug mode headers. Instead of only enabling them in VS2013's debug mode,
   # we'll just enable them for Visual Studio 2015 (VS 14, MSVC_VERSION 1900)
   # and up.
   if (NOT (MSVC_VERSION LESS 1900))
     # Disable string literal const->non-const type conversion.
     # "When specified, the compiler requires strict const-qualification
     # conformance for pointers initialized by using string literals."