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

Also makes -fexec-charset accept utf-8 case-insensitively.
Like https://reviews.llvm.org/D23807, but for execution-charset.
Also replace a few .lower() comparisons with equals_lower().

https://reviews.llvm.org/D23938

llvm-svn: 279866