[MIR] Target specific MIR formating and parsing

Summary:
Added MIRFormatter for target specific MIR formating and parsing with
immediate and custom pseudo source values. Target machine can subclass
MIRFormatter and implement custom logic for printing and parsing
immediate and custom pseudo source values for better readability.

* Target specific immediate mnemonic need to start with "." follows by
  identifier string. When MIR parser sees immediate it will call target
  specific parsing function.

* Custom pseudo source value need to start with custom follows by
  double-quoted string. MIR parser will pass the quoted string to target
  specific PSV parsing function.

* MIRFormatter have 2 helper functions to facilitate LLVM value printing
  and parsing for custom PSV if they refers LLVM values.

Patch by Peng Guo

Reviewers: dsanders, arsenm

Reviewed By: dsanders

Subscribers: wdng, jvesely, nhaehnle, hiraditya, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69836