LowerBitSets: Extend pass to support functions as bitset members.

This change extends the bitset lowering pass to support bitsets that may
contain either functions or global variables. A function bitset is lowered to
a jump table that is laid out before one of the functions in the bitset.

Also add support for non-string bitset identifier names. This allows for
distinct metadata nodes to stand in for names with internal linkage,
as done in D11857.

Differential Revision: http://reviews.llvm.org/D11856

llvm-svn: 247080