Bug 1222166 - use gcc/clang warning flags for clang-cl in moz.build files; r=glandium

This patch turns off a number of gcc/clang-specific warnings for
clang-cl as well, as clang-cl understands all the warning flags that
clang understands.  We currently don't turn on all the gcc/clang
warnings for clang-cl in configure, but that can be done separately, and
this patch addresses some pain points (particularly for cairo).