LowerTypeTests: Drop function type metadata only if we're going to replace it.

Previously we were (mis)handling jump table members with a prevailing
definition in a full LTO module and a non-prevailing definition in a
ThinLTO module by dropping type metadata on those functions entirely,
which would cause type tests involving such functions to fail.

This patch causes us to drop metadata only if we are about to replace
it with metadata from cfi.functions.

We also want to replace metadata for available_externally functions,
which can arise in the opposite scenario (prevailing ThinLTO
definition, non-prevailing full LTO definition). The simplest way
to handle that is to remove the definition; there's little value in
keeping it around at this point (i.e. after most optimization passes
have already run) and later code will try to use the function's linkage
to create an alias, which would result in invalid IR if the function
is available_externally.

Fixes PR33832.

Differential Revision: https://reviews.llvm.org/D35604

llvm-svn: 308642