Slightly simplify MappedBlockStream::createIndexedStream() calls

All callers had a PDBFile object at hand, so call
Pdb.createIndexedStream() instead, which pre-populates all the arguments
(and returns nullptr for kInvalidStreamIndex).

Also change safelyCreateIndexedStream() to only take the string index,
and update callers. Make the method public and call it in two places
that manually did the bounds checking before.

No intended behavior change.

Differential Revision: https://reviews.llvm.org/D64633

llvm-svn: 365936