[Transforms] Merge function attributes within InlineFunction (NFC)

In the past, we've had a bug resulting in a compiler crash after
forgetting to merge function attributes (D105729).

This patch teaches InlineFunction to merge function attributes.  This
way, we minimize the "time" when the IR is valid, but the function
attributes are not.

Differential Revision: https://reviews.llvm.org/D134117