Start handling debug line and scope information better:

    Migrate most of the location setting within the larger API in CGDebugInfo and
    update a lot of callers.
    Remove the existing file/scope change machinery in UpdateLineDirectiveRegion
    and replace it with DILexicalBlockFile usage.

    Finishes off the rest of rdar://10246360

after fixing a few bugs that were exposed in gdb testsuite testing.

llvm-svn: 141893