[pdb] Have builders share a single BumpPtrAllocator.

This makes it easier to have the writable and readable PDB
interfaces share code since the read/write and write-only
interfaces now share a single allocator, you don't have to worry
about a builder building a read only interface and then having
the read-only interface's data become corrupt when the builder
goes out of scope.  Now the allocator is specified explicitly
to all constructors, so all interfaces can share a single allocator
that is scoped appropriately.

llvm-svn: 276459