Simplify some va_start checking logic

Combine the logic doing the ms_abi/sysv_abi checks into one function so
that each check and its logical opposite are near each other. Now we
don't need two Sema entry points for MS va_start and regular va_start.

Refactor the code that checks if the va_start caller is a function,
block, or obj-c method. We do this in three places, and they are all
buggy for variadic lambdas (PR32737). After this change, I have one
place to apply the functional fix.

NFC

llvm-svn: 301968