Add support for assigning to . in AsmParser.

This is implemented by handling assignments to the '.' pseudo symbol
as ".org" directives.

Differential Revision: http://llvm-reviews.chandlerc.com/D2625



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201530 91177308-0d34-0410-b5e6-96231b3b80d8