clang-cl: Support MSVC2015's /validate-charset flag.

Clang always assumes that files are utf-8. If an invalidly encoded character is
used in an identifier, clang always errors. If it's used in a character
literal, clang warns Winvalid-source-encoding (on by default). Clang never
checks the encoding of things in comments (adding this seems like a nice
feature if it doesn't impact performance).

For cl.exe /utf-8 (which enables /validate-charset), if a bad character is used
in an identifier, it emits both an error and a warning. If it's used in a
literal or a comment, it emits a warning.

So mapping /validate-charset to -Winvalid-source-encoding seems like a fairly
decent fit.

https://reviews.llvm.org/D23945

llvm-svn: 279872