[Support] Remove incorrect noalias return attribute in BumpPtrAllocator

The memory returned by the Allocate() function is also otherwise
accessible -- and is indeed accessed by the DestroyAll() method of
SpecificBumpPtrAlloactor. This is a violation of the noalias return
contract.

This should address the issue reported in
https://reviews.llvm.org/D116728#3252464.

Differential Revision: https://reviews.llvm.org/D117664