[mips] Defer validity check for CPU/ABI pairs and improve error message for invalid cases.

Summary:
The validity of ABI/CPU pairs is no longer checked on the fly but is
instead checked after initialization. As a result, invalid CPU/ABI pairs
can be reported as being known but invalid instead of being unknown. For
example, we now emit:
  error: ABI 'n32' is not supported on CPU 'mips32r2'
instead of:
  error: unknown target ABI 'n64'

Reviewers: atanasyan

Subscribers: sdardis, cfe-commits

Differential Revision: http://reviews.llvm.org/D21023

llvm-svn: 272645