MC/AsmParser: Add support for 'a + 4@GOTPCREL' and friends, by reconsing the

MC/AsmParser: Add support for 'a + 4@GOTPCREL' and friends, by reconsing the
expression to include the modifier.
 - Gross, but this a corner case we don't expect to see often in practice, but
   it is worth accepting.
 - Also improves diagnostics on invalid modifiers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@114154 91177308-0d34-0410-b5e6-96231b3b80d8