[mips] Replace custom parsing logic for data directives by the…


[mips] Replace custom parsing logic for data directives by the addAliasForDirective

The target independent AsmParser doesn't recognise .hword, .word, .dword
which are required for Mips. Currently MipsAsmParser recognises these
through dispatch to MipsAsmParser::parseDataDirective. This contains
equivalent logic to AsmParser::parseDirectiveValue. This patch allows
reuse of AsmParser::parseDirectiveValue by making use of
addAliasForDirective to support .hword, .word and .dword.

Original patch provided by Alex Bradbury at D47001 was modified to fix
handling of microMIPS symbols. The AsmParser::parseDirectiveValue
calls either EmitIntValue or EmitValue. In this patch we override
EmitIntValue in the MipsELFStreamer to clear a pending set of
microMIPS symbols.

Differential revision: https://reviews.llvm.org/D49539