[-Wunsafe-buffer-usage] Refactor and improve for parameter fix-its

- Factor out the code that will be shared by both parameter and local variable fix-its
- Add a check to ensure that a TypeLoc::isNull is false before using the TypeLoc
- Remove the special check for whether a fixing variable involves unnamed types. This check is unnecessary now.
- Move tests for cv-qualified parameters and unnamed types out of the "...-unsupported.cpp" test file.

Reviewed by: NoQ (Artem Dergachev)

Differential revision: https://reviews.llvm.org/D156188