[AsmParser] Add DiagnosticString to AsmOperands in tablegen

This adds a DiagnosticString member to the AsmOperand tablegen class, so
that the diagnostic text to be used when an assembly operand is
incorrect can be stored in the tablegen description of the operand,
rather than in a separate switch statement in the AsmParser.

If DiagnosticString is used for any operands, tablegen will emit a
getMatchKindDiag function, to map from diagnostic enums to strings.

Differential revision: https://reviews.llvm.org/D31606

llvm-svn: 314803