[mlir:PassOptions] Fix parsing of nested option values/better handle escaping

The option parser currently does not properly handle nested options, meaning that
in some cases we can print pass pipelines that we can't actually parse back in. For example,
from #52885 we currently can't parse in inliner pipelines that have nested pipeline strings.

This commit adds handling for string values (e.g. "...") and nested options
(e.g. `foo{baz{bar=10 pizza=11}}`).

Fixes #52885

Differential Revision: https://reviews.llvm.org/D118078