[MemoryBuiltins] Remove isNoAliasFn() in favor of isNoAliasCall()

We currently have two similar implementations of this concept:
isNoAliasCall() only checks for the noalias return attribute.
isNoAliasFn() also checks for allocation functions.

We should switch to only checking the attribute. SLC is responsible
for inferring the noalias return attribute for non-new allocation
functions (with a missing case fixed in
https://github.com/llvm/llvm-project/commit/348bc76e3548c52dbcd442590ca0a7f5b09b7534).
For new, clang is responsible for setting the attribute,
if -fno-assume-sane-operator-new is not passed.

Differential Revision: https://reviews.llvm.org/D116800