[clang-cl] Implement /diagnostic: flag

This flag has three possible values: caret, column, and classic.

"caret" corresponds to clang's default mode, "column" removes the caret
and code snippet, and "classic" emits nothing.

Documentation is here:
https://docs.microsoft.com/en-us/cpp/build/reference/diagnostics-compiler-diagnostic-options

Implements the last part of PR33237

llvm-svn: 304316