[Fortran Support] Detect Fortran arrays & metadata from dragonegg output

Add the ability to tag certain memory accesses as those belonging to
Fortran arrays. We do this by pattern matching against known patterns
of Dragonegg's LLVM IR output from Fortran code.

Fortran arrays have metadata stored with them in a struct. This struct
is called the "Fortran array descriptor", and a reference to this is
stored in each MemoryAccess.

Differential Revision: https://reviews.llvm.org/D32639

llvm-svn: 302653