DebugInfo: Fix testcases that fail -verify-debug-info=true

As part of PR22777, fix testcases that fail the debug info verifier.
The changes fall into the following categories:

  - Empty `filename:` fields in `MDFile`s.  Compile units and some types
    require non-empty filenames.  A number of testcases have empty
    filenames, probably due to hand-reduction of testcases.
  - Not-quite empty arrays: `!{i32 0}`.  This used to be equivalent in
    the debug info schema to `!{}`.  They cause problems for
    `!MDSubroutineType`'s `types:` array, since it requires all operands
    to be valid types.  (Note that `!{null}` is the correct type array
    for functions that take no arguments and return `void`.)
  - Significantly bitrotted testcases.  Nodes got left behind a few
    upgrades ago because of missing or invalid tags.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232415 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/ArgumentPromotion/dbg.ll

@@ -22,5 +22,6 @@ define void @caller(i32** %Y) {
 !0 = !{i32 2, !"Debug Info Version", i32 3}
 !1 = !MDLocation(line: 8, scope: !2)
 !2 = !MDSubprogram(name: "test", line: 3, isLocal: true, isDefinition: true, virtualIndex: 6, flags: DIFlagPrototyped, isOptimized: false, scopeLine: 3, scope: null, function: void (i32**)* @test)
-!3 = !MDCompileUnit(language: DW_LANG_C_plus_plus, producer: "clang version 3.5.0 ", isOptimized: false, emissionKind: 2, file: null, subprograms: !4)
+!3 = !MDCompileUnit(language: DW_LANG_C_plus_plus, producer: "clang version 3.5.0 ", isOptimized: false, emissionKind: 2, file: !5, subprograms: !4)
 !4 = !{!2}
+!5 = !MDFile(filename: "test.c", directory: "")