Bug 7440 7458: Update to LLVMDebugVersion 12

* sources/lib/llvm/llvm-debug.dylan
  ($llvm-debug-version): Update to version 12.
  (llvm-make-dbg-compile-unit): Change metadata layout to match version 12.
  (llvm-make-dbg-file-directory): New internal utility function.
  (llvm-make-dbg-file): Change metadata layout to match version 12.
  (as-file-directory): New internal utility function
  (llvm-make-dbg-function-type, llvm-make-dbg-function,
   llvm-make-dbg-lexical-block, llvm-make-dbg-basic-type,
   llvm-make-dbg-derived-type, llvm-make-dbg-composite-type): Change
   metadata layout to match version 12.
* sources/lib/llvm/tests/llvm-builder-test.dylan
  (function-test ins--dbg): Update to new llvm-make-dbg-file signature.
* sources/dfmc/llvm-back-end/llvm-emit-debug.dylan
  (llvm-source-record-dbg-file): Update to new llvm-make-dbg-file signature.