[mlir] Add support for regex within `expected-*` diagnostics

This can be enabled by using a `-re` suffix when defining the expected line,
e.g. `expected-error-re`. This support is similar to what clang provides in its "expected"
diagnostic framework(e.g. the `-re` is also the same). The regex definitions themselves are
similar to  FileCheck in that regex blocks are specified within `{{` `}}` blocks.

Differential Revision: https://reviews.llvm.org/D129343