BitcodeReader: Use getMD/getMDOrNull helpers consistently, almost NFC

The only functionality change was removing an error check from the
BitcodeReader (and an assertion from DILocation::getImpl) that is
already caught by Verifier::visitDILocation.  The Verifier is a better
place for this anyway, and being inconsistent with other subclasses of
MDNode isn't serving anyone.

llvm-svn: 267267