[AsmMatcher] Extend PredicateMethod with optional DiagnosticPredicate

An optional, light-weight and backward-compatible mechanism to allow
specifying that a diagnostic _only_ applies to a partial mismatch (NearMiss),
rather than a full mismatch.

Patch [1/2] in a series to improve assembler diagnostics for SVE.
-  Patch [1/2]: https://reviews.llvm.org/D45879
-  Patch [2/2]: https://reviews.llvm.org/D45880

Reviewers: olista01, stoklund, craig.topper, mcrosier, rengolin, echristo, fhahn, SjoerdMeijer, evandro, javed.absar

Reviewed By: olista01

Differential Revision: https://reviews.llvm.org/D45879

llvm-svn: 330930