clang-format: Add support for SEH __try / __except / __finally blocks.

This lets clang-format format

    __try {
    } __except(0) {
    }

and

    __try {
    } __finally {
    }

correctly. __try and __finally are keywords if `LangOpts.MicrosoftExt` is set,
so this turns this on.  This also enables a few other keywords, but it
shouldn't overly perturb regular clang-format operation.  __except is a
context-sensitive keyword, so `AdditionalKeywords` needs to be passed around to
a few more places.

Fixes PR22321.

llvm-svn: 228148