[clang] NFC: refactor multiple implementations of getDecltypeForParenthesizedExpr

This cleanup patch refactors a bunch of functional duplicates of
getDecltypeForParenthesizedExpr into a common implementation.

Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>

Reviewed By: aaronpuchert

Differential Revision: https://reviews.llvm.org/D100713