Compute some Debug Info Metadata hash key partially (NFC)

Summary:
This patch changes the computation of the hash key for DISubprogram to
be computed on a small subset of the fields. The hash is computed a
lot faster, but there might be more collision in the table.
However by carefully selecting the fields, colisions should be rare.

Using `opt` to load the IR for FastISelEmitter.cpp.o, with this patch:
 - DISubprogram::getImpl() goes from 28ms to 15ms.
 - DICompositeType::getImpl() goes from 6ms to 2ms
 - DIDerivedType::getImpl() goes from 18 to 12ms

Reviewers: dexonsmith

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D16571

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 263866