[OpenMP][FIX] Do not overalign mapped structures

While we potentially need to align partially mapped structs more than
the first member, we do not need to align past the struct itself. This
prevents us from moving the base pointer past the struct beginning too.

See https://reviews.llvm.org/D142508 for a discussion.

Reviewed By: pavelkopyl, grokos, jhuber6

Differential Revision: https://reviews.llvm.org/D142586