[C++20] [Modules] Make placement allocation functions always visible

Close https://github.com/llvm/llvm-project/issues/59601.

This is actually a workaround for the issue. See the comments and the
test for example. The proper fix should make the placement allocation
functions acceptable based on the context. But it is harder and more
complex on the one side. On the other side, such workaround won't be too
bad in practice since users rarely call the placement allocation
functions directly.

So personally I prefer to address such problems in the simpler way.

Reviewed By: royjacobson

Differential Revision: https://reviews.llvm.org/D141023