[msf] Make FPM reader use MappedBlockStream.

MappedBlockSTream can work with any sequence of block data where
the ordering is specified by a list of block numbers.  So rather
than manually stitch them together in the case of the FPM, reuse
this functionality so that we can treat the FPM as if it were
contiguous.

Reviewed By: ruiu
Differential Revision: https://reviews.llvm.org/D23066

llvm-svn: 277609