[PATCH]: Remove DW_TAG_arg_variable and DW_TAG_auto_variable

[Duncan P. N. Exon Smith]

These patches remove the two remaining fake DWARF tags, which we use to
differentiate between types of variables.  However, they're not
necessary.  The `arg:` field fully encodes whether a local variable is a
parameter.

The other obvious option (and my original plan here) was to create two
subclasses `DIAutoVariable` and `DIParameterVariable`.  These classes
would be so similar that I'm not sure that's worthwhile until we're
tablegen'ing these.

Here's what's in the patches:

  - 0001: Update the DIBuilder API to use `createAutoVariable()` and
    `createParameterVariable()` (instead of `createLocalVariable()`).
  - 0002: clang side of 0001.
  - 0003: Strip the fake tags from the IR.
  - 0004: LLVM testcases for 0003 updated via script (I'll squash this).
  - 0005: Clang testcases for 0003.

(Note: a follow-up could change `DILocalVariable::DILocalVariable()` to
set the tag to `DW_TAG_formal_parameter` instead of `DW_TAG_variable`
(as appropriate), instead of having that logic magically in the backend
in `DbgVariable`.  I left a FIXME to that effect.)


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