IR: Change MDFile to directly store the filename/directory

IR: Change MDFile to directly store the filename/directory

In the old (well, current) schema, there are two types of file
references: untagged and tagged (the latter references the former).

    !0 = !{!"filename", !"/directory"}
    !1 = !{!"0x29", !1} ; DW_TAG_file_type [filename] [/directory]

The interface to `DIBuilder` universally takes the tagged version,
described by `DIFile`.  However, most `file:` references actually use
the untagged version directly.

In the new hierarchy, I'm merging this into a single node: `MDFile`.

Originally I'd planned to keep the old schema unchanged until after I
moved the new hierarchy into place.

However, it turns out to be trivial to make `MDFile` match both nodes at
the same time.

  - Anyone referencing !1 does so through `DIFile`, whose implementation
    I need to gut anyway (as I do the rest of the `DIDescriptor`s).
  - Anyone referencing !0 just references an `MDNode`, and expects a
    node with two `MDString` operands.

This commit achieves that, and updates all the testcases for the parts
of the new hierarchy that used the two-node schema (I've replaced the
untagged nodes with `distinct !{}` to make the diff clear (otherwise the
metadata all gets renumbered); it might be worthwhile to come back and
delete those nodes and renumber the world, not sure).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230057 91177308-0d34-0410-b5e6-96231b3b80d8