[mips] Defer validity check for CPU/ABI pairs and improve error message for invalid cases.
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'
error: unknown target ABI 'n64'
Subscribers: sdardis, cfe-commits
Differential Revision: http://reviews.llvm.org/D21023