Remove asserting getters from base Type

Summary:
Remove asserting vector getters from Type in preparation for the
VectorType refactor. The existence of these functions complicates the
refactor while adding little value.

Reviewers: dexonsmith, sdesmalen, efriedma

Reviewed By: efriedma

Subscribers: cfe-commits, hiraditya, llvm-commits

Tags: #llvm, #clang

Differential Revision: https://reviews.llvm.org/D77278