[windows] Add /Gw to compiler flags

[thakis]

Summary

This is like -fdata-sections, and it's not part of /O2 by default for some reason.

In the cmake build, reduces the size of clang.exe from 70,358,016 bytes to 69,982,720 bytes.

clang-format.exe goes from 3,703,296 bytes to 3,331,072 bytes.

[rnk]

lgtm

llvm/cmake/modules/HandleLLVMOptions.cmake

if (C_SUPPORTS_FNO_FUNCTION_SECTIONS)
       # Don't add -ffunction-sections if it can't be disabled with -fno-function-sections.
       # Doing so will break sanitizers.
       add_flag_if_supported("-ffunction-sections" FFUNCTION_SECTIONS)
     elseif (CMAKE_CXX_COMPILER_ID MATCHES "XL")
       append("-qfuncsect" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
     endif()
     add_flag_if_supported("-fdata-sections" FDATA_SECTIONS)
   endif()
+elseif(MSVC)
+  if( NOT uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG" )
+    append("/Gw" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+  endif()
 endif()
 
 if(MSVC)
   # Remove flags here, for exceptions and RTTI.
   # Each target property or source property should be responsible to control
   # them.
   # CL.EXE complains to override flags like "/GR /GR-".
   string(REGEX REPLACE "(^| ) */EH[-cs]+ *( |$)" "\\1 \\2" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")

llvm/utils/gn/build/BUILD.gn

if (is_debug) {
       cflags += [
         "/Zi",
         "/FS",
       ]
       ldflags += [ "/DEBUG" ]
     }
     if (is_optimized) {
       cflags += [
-        # FIXME: evaluate /Gw (not part of /O2)
         "/O2",
+        "/Gw",
         "/Zc:inline",
       ]
       ldflags += [
         "/OPT:REF",
         "/OPT:ICF",
       ]
     }
     defines += [