[ms] [llvm-ml] Improve MASM STRUCT field accessor support

Summary:
Adds support for several accessors:
- `[<identifier>.<struct name>].<field>`
- `[<identifier>.<struct name>.<field>].<subfield>` (where `field` has already-defined STRUCT type)
- `[<variable>.<field>].<subfield>` (where `field` has already-defined STRUCT type)

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D83344