[codeview] Clean up machinery for deferring .cv_loc emission

Now that we create the label at the point of the directive, we don't
need to set the "current CV location", and then later when we emit the
next instruction, create a label for it and emit it.

DWARF still defers the labels used in .debug_loc until the next
instruction or value, for reasons unknown.

llvm-svn: 340883