[mips] Rename [gs]etCanHaveModuleDir to more natural names

Summary:
getCanHaveModuleDir() is renamed to isModuleDirectiveAllowed(), and
setCanHaveModuleDir() is renamed to forbidModuleDirective() since it is only
ever given a false argument.

Reviewers: vmedic

Reviewed By: vmedic

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D4885