[mlir] Refactor the Parser library in preparation for an MLIR binary format

The current Parser library is solely focused on providing API for
the textual MLIR format, but MLIR will soon also provide a binary
format. This commit renames the current Parser library to AsmParser to
better correspond to what the library is actually intended for. A new
Parser library is added which will act as a unified parser interface
between both text and binary formats. Most parser clients are
unaffected, given that the unified interface is essentially the same as
the current interface. Only clients that rely on utilizing the
AsmParserState, or those that want to parse Attributes/Types need to be
updated to point to the AsmParser library.

Differential Revision: https://reviews.llvm.org/D129605