win: Pass /W4 in front of all the -wd flags.

[thakis]

Summary

This should fix many many -Wunused-parameter warnings in self-host builds on Windows after r255382.

[thakis]

more direct

[thakis]

V2 of this seems simple enough to land it for post-commit review. Landed this in 256315.

[rnk]

Yep, lgtm.

cmake/modules/HandleLLVMOptions.cmake

# specifier (MSVC 2015?) this warning can be re-enabled.
     -w14062 # Promote 'enumerator in switch of enum is not handled' to level 1 warning.
 
     # Promoted warnings to errors.
     -we4238 # Promote 'nonstandard extension used : class rvalue used as lvalue' to error.
     )
 
   # Enable warnings
   if (LLVM_ENABLE_WARNINGS)
-    append("/W4" msvc_warning_flags)
+    # Put /W4 in front of all the -we flags. cl.exe doesn't care, but for
+    # clang-cl having /W4 after the -we flags will re-enable the warnings
+    # disabled by -we.
+    set(msvc_warning_flags "/W4 ${msvc_warning_flags}")
     # CMake appends /W3 by default, and having /W3 followed by /W4 will result in 
     # cl : Command line warning D9025 : overriding '/W3' with '/W4'.  Since this is
     # a command line warning and not a compiler warning, it cannot be suppressed except
     # by fixing the command line.
     string(REGEX REPLACE " /W[0-4]" "" CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
     string(REGEX REPLACE " /W[0-4]" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
 
     if (LLVM_ENABLE_PEDANTIC)