MC: Allow modifiers in MCSymbolRefExpr, and eliminate X86MCTargetExpr.

 - Although it would be nice to allow this decoupling, the assembler needs to be able to reason about MCSymbolRefExprs in too many places to make this viable. We can use a target specific encoding of the variant if this becomes an issue.
 - This patch also extends llvm-mc to support parsing of the modifiers, as opposed to lumping them in with the symbol.

llvm-svn: 98592