[-Wunsafe-buffer-usage] Stop generating incorrect fix-its for variable...

[-Wunsafe-buffer-usage] Stop generating incorrect fix-its for variable declarations with unsupported specifiers

We have to give up on fixing a variable declaration if it has
specifiers that are not supported yet.  We could support these
specifiers incrementally using the same approach as how we deal with
cv-qualifiers. If a fixing variable declaration has a storage
specifier, instead of trying to find out the source location of the
specifier or to avoid touching it, we add the keyword to a
canonicalized place in the fix-it text that replaces the whole
declaration.

Reviewed by: NoQ (Artem Dergachev), jkorous (Jan Korous)

Differential revision: https://reviews.llvm.org/D156192