[support] GlobPattern: add support for `\` and `[!...]`, and allow `]` in more places

Summary: Update GlobPattern in libSupport to handle a few more cases. It does not fully match the `fnmatch` used by GNU objcopy since named character classes (e.g. `[[:digit:]]`) are not supported, but this should support most existing use cases (mostly just `*` is what's used anyway).

This will be used to implement the `--wildcard` flag in llvm-objcopy to be more compatible with GNU objcopy.

This is split off of D66613 to land the libSupport changes separately. The llvm-objcopy part will land soon.

Reviewers: jhenderson, MaskRay, evgeny777, espindola, alexshap

Reviewed By: MaskRay

Subscribers: nickdesaulniers, emaste, arichardson, hiraditya, jakehehrlich, abrachet, seiya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66613

undo objcopy changes to make this libsupport only

llvm-svn: 375051