[VE] Support symbol with offset in assembly

Summary:
Change MCExpr to support Aurora VE's modifiers.  Change asmparser to use
existing MCExpr parser (parseExpression) to parse an expression contining
symbols with modifiers and offsets.  Also add several regression tests
of MC layer.

Reviewers: simoll, k-ishizaka

Reviewed By: simoll

Subscribers: hiraditya, llvm-commits

Tags: #llvm, #ve

Differential Revision: https://reviews.llvm.org/D83170