[build.py] Allow clang-cl to build files starting with '/U'
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.
Differential Revision: https://reviews.llvm.org/D58860