Remove the (apparently) unnecessary debug info metadata indirection.

The main lists of debug info metadata attached to the compile_unit had an extra
layer of metadata nodes they went through for no apparent reason. This patch
removes that (& still passes just as much of the GDB 7.5 test suite). If anyone
can show evidence as to why these extra metadata nodes are there I'm open to
reverting this patch & documenting why they're there.

llvm-svn: 174266