[llvm-readelf] Allow single-letter flags to be merged.

Summary:
This patch adds support for merged arguments (e.g. -SW == -S -W) for llvm-readelf.

No changes are intended for llvm-readobj. There are a few short flags (-sd, -sr, -st, -dt) that would conflict with grouped single letter flags, and having only some grouped flags might be confusing. So, allow merged flags for readelf compatibility, but force separate args for llvm-readobj. From what I can tell, these two-letter flags are only used with llvm-readobj, not llvm-readelf.

This fixes PR40064.

Reviewers: jhenderson, kristina, echristo, phosek

Reviewed By: jhenderson

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D56629

llvm-svn: 351205