MemoryBuiltins: also check function definition for allocalign

This got changed to use hasAttrSomewhere() during review, and I didn't
notice until today when I was writing some tests for another part of
this system that using hasAttrSomewhere only checked the callsite for
allocalign, rather than both the callsite and the definition. This fixes
that by introducing a helper method.

Differential Revision: https://reviews.llvm.org/D121641