[windows] Add /Gw to compiler flags

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.

Differential Revision: https://reviews.llvm.org/D74573

cmake/modules/HandleLLVMOptions.cmake

@@ -793,6 +793,10 @@ if(NOT CYGWIN AND NOT WIN32)
     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)

utils/gn/build/BUILD.gn

@@ -59,8 +59,8 @@ config("compiler_defaults") {
     }
     if (is_optimized) {
       cflags += [
-        # FIXME: evaluate /Gw (not part of /O2)
         "/O2",
+        "/Gw",
         "/Zc:inline",
       ]
       ldflags += [