Constify the argument to SourceLocation::getFromPtrEncoding.

This allows SourceLocations to be stored in generic "data" fields
that are typed as "const void *" and are also used to point to
const objects.

Really we should probably be returning a const pointer from
getPtrEncoding as well, but in some places we want to store
SourceLocations in the same generic "data" field as proper
pointers to /mutable/ objects. Oh well.

llvm-svn: 159868