[Hexagon] Use addAliasForDirective for data directives

[Hexagon] Use addAliasForDirective for data directives

Data directives such as .word, .half, .hword are currently parsed using
HexagonAsmParser::ParseDirectiveValue which effectively duplicates logic from
AsmParser::parseDirectiveValue. This patch deletes that duplicated logic in
favour of using addAliasForDirective.

Differential Revision: https://reviews.llvm.org/D46999

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332607 91177308-0d34-0410-b5e6-96231b3b80d8