[RISCV] Fix failure to parse parenthesized immediates

Since the parser attempts to parse an operand as a register with
parentheses before parsing it as an immediate, immediates in
parentheses should not be parsed by parseRegister. However in the case
where the immediate does not start with an identifier, the LParen is not
unlexed and so the RParen causes an unexpected token error.

This patch adds the missing UnLex, and modifies the existing UnLex to
not use a buffered token, as it should always be unlexing an LParen.

Differential Revision: https://reviews.llvm.org/D57319

llvm-svn: 363782