PDBFPO: Use references instead of pointers, where possible

Summary:
The code was passing pointers around, expecting they would be not null.
In c++ it is possible to convey this notion explicitly by using a
reference instead.

Not all uses of pointers could be converted to references (e.g. one
can't store references in a container), but this will at least make it
locally obvious that code is dealing with nonnull pointers.

Reviewers: aleksandr.urakov, amccarth

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D60271

llvm-svn: 357744