full asmparser support for blockaddress. We can now do:

full asmparser support for blockaddress.  We can now do:
$ llvm-as foo.ll -d -disable-output

which reads and prints the .ll file.  BC encoding is the
next project.  Testcase will go in once that works.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@85368 91177308-0d34-0410-b5e6-96231b3b80d8