clang-cl: Accept MSVC 2015's `/source-charset:utf-8` flag.

clang already treats all inputs as utf-8. Warn if anything but utf-8 is passed.

Do this by mapping source-charset to finput-charset, which already behaves like
this. Slightly tweak finput-charset to accept "utf-8" case-insensitively. This
matches gcc's and cl.exe's behavior, and IANA says that character set names are
case-insensitive.

https://reviews.llvm.org/D23807

llvm-svn: 279531