LLParser: Handle BlockAddresses on-the-fly

Previously all `blockaddress()` constants were treated as forward
references.  They were resolved twice:  once at the end of the function
in question, and again at the end of the module.  Furthermore, if the
same blockaddress was referenced N times, the parser created N distinct
`GlobalVariable`s (one for each reference).

Instead, resolve all block addresses at the beginning of the function,
creating the standard `BasicBlock` forward references used for all other
basic block references.  After the function, all references can be
resolved immediately.  To check for the condition of parsing block
addresses from within the same function, I created a reference to the
current per-function-state in `BlockAddressPFS`.

Also, create only one forward-reference per basic block.  Because
forward references to block addresses are rare, the data structure here
shouldn't matter.  If somehow it does someday, this can be pretty easily
changed to a `DenseMap<std::pair<ValID, ValID>, GV>`.

This is part of PR20515.

llvm-svn: 215952