[build.py] Allow clang-cl to build files starting with '/U'

[xiaobai]

Summary

clang-cl tries to match cl's interface, and treats /U as "Removes a
predefined macro" as cl does. When you feed clang-cl a file that begins with
'/U' (e.g. /Users/xiaobai/foo.c), clang-cl will emit a warning and in some cases
an error, like so:

clang-9: warning: '/Users/xiaobai/foo.c' treated as the '/U' option [-Wslash-u-filename]
clang-9: note: Use '--' to treat subsequent arguments as filenames
clang-9: error: no input files

If you're using clang-cl, make sure '--' is passed before the source file.

[labath]

Seems straight-forward enough.

lit/helper/build.py

def _get_compilation_command(self, source, obj):
         args.append('/Z7')
         if self.toolchain_type == 'clang-cl':
             args.append('-Xclang')
             args.append('-fkeep-static-consts')
             args.append('-fms-compatibility-version=19')
         args.append('/c')
 
         args.append('/Fo' + obj)
+        if self.toolchain_type == 'clang-cl':
+            args.append('--')
         args.append(source)
 
         return ('compiling', [source], obj,
                 self.compile_env,
                 args)
 
     def _get_link_command(self):
         args = []