[ms] [llvm-ml] Fix struct padding logic

MASM structs are end-padded to have size a multiple of the smaller of the requested alignment and the size of their largest field (taken recursively, if they have a field of STRUCT type).

This matches the behavior of ml.exe and ml64.exe. Our original implementation followed the MASM 6.0 documentation, which instead specified that MASM structs were padded to a multiple of their requested alignment.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D87248