Since we're stuck with realpath for the header <-> module mapping,
factor the realpath calls into FileManager::getCanonicalName() so we
can cache the results of this epically slow operation. 5% speedup on
my modules test, and realpath drops out of the profile.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173542 91177308-0d34-0410-b5e6-96231b3b80d8