DI: Remove DW_TAG_arg_variable and DW_TAG_auto_variable

Remove the fake `DW_TAG_auto_variable` and `DW_TAG_arg_variable` tags,
using `DW_TAG_variable` in their place Stop exposing the `tag:` field at
all in the assembly format for `DILocalVariable`.

Most of the testcase updates were generated by the following sed script:

    find test/ -name "*.ll" -o -name "*.mir" |
    xargs grep -l 'DILocalVariable' |
    xargs sed -i '' \
      -e 's/tag: DW_TAG_arg_variable, //' \
      -e 's/tag: DW_TAG_auto_variable, //'

There were only a handful of tests in `test/Assembly` that I needed to
update by hand.

(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've added a FIXME to that effect.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243774 91177308-0d34-0410-b5e6-96231b3b80d8

test/Bitcode/DILocalVariable-explicit-tags.ll

@@ -0,0 +1,16 @@
+; Bitcode compiled with 3.7_rc2.  3.7 had redundant (but mandatory) tag fields
+; on DILocalVariable.
+;
+; RUN: llvm-dis < %s.bc -o - | llvm-as | llvm-dis | FileCheck %s
+
+; CHECK: ![[SP:[0-9]+]] = !DISubprogram(name: "foo",{{.*}} variables: ![[VARS:[0-9]+]]
+; CHECK: ![[VARS]] = !{![[PARAM:[0-9]+]], ![[AUTO:[0-9]+]]}
+; CHECK: ![[PARAM]] = !DILocalVariable(name: "param", arg: 1, scope: ![[SP]])
+; CHECK: ![[AUTO]]  = !DILocalVariable(name: "auto", scope: ![[SP]])
+
+!named = !{!0}
+
+!0 = !DISubprogram(name: "foo", variables: !1)
+!1 = !{!2, !3}
+!2 = !DILocalVariable(tag: DW_TAG_arg_variable, name: "param", arg: 1, scope: !0)
+!3 = !DILocalVariable(tag: DW_TAG_auto_variable, name: "auto", scope: !0)