DI: Disallow uniquable DICompileUnits

Since r241097, `DIBuilder` has only created distinct `DICompileUnit`s.
The backend is liable to start relying on that (if it hasn't already),
so make uniquable `DICompileUnit`s illegal and automatically upgrade old
bitcode.  This is a nice cleanup, since we can remove an unnecessary
`DenseSet` (and the associated uniquing info) from `LLVMContextImpl`.

Almost all the testcases were updated with this script:

    git grep -e '= !DICompileUnit' -l -- test |
    grep -v test/Bitcode |
    xargs sed -i '' -e 's,= !DICompileUnit,= distinct !DICompileUnit,'

I imagine something similar should work for out-of-tree testcases.

llvm-svn: 243885
diff --git a/llvm/test/DebugInfo/X86/debug-loc-offset.ll b/llvm/test/DebugInfo/X86/debug-loc-offset.ll
index f2a5556..f912708 100644
--- a/llvm/test/DebugInfo/X86/debug-loc-offset.ll
+++ b/llvm/test/DebugInfo/X86/debug-loc-offset.ll
@@ -116,7 +116,7 @@
 !llvm.module.flags = !{!18, !19}
 !llvm.ident = !{!20, !20}
 
-!0 = !DICompileUnit(language: DW_LANG_C_plus_plus, producer: "clang version 3.5.0 (210479)", isOptimized: false, emissionKind: 1, file: !1, enums: !2, retainedTypes: !2, subprograms: !3, globals: !2, imports: !2)
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, producer: "clang version 3.5.0 (210479)", isOptimized: false, emissionKind: 1, file: !1, enums: !2, retainedTypes: !2, subprograms: !3, globals: !2, imports: !2)
 !1 = !DIFile(filename: "debug-loc-offset1.cc", directory: "/llvm_cmake_gcc")
 !2 = !{}
 !3 = !{!4}
@@ -125,7 +125,7 @@
 !6 = !DISubroutineType(types: !7)
 !7 = !{!8, !8}
 !8 = !DIBasicType(tag: DW_TAG_base_type, name: "int", size: 32, align: 32, encoding: DW_ATE_signed)
-!9 = !DICompileUnit(language: DW_LANG_C_plus_plus, producer: "clang version 3.5.0 (210479)", isOptimized: false, emissionKind: 1, file: !10, enums: !2, retainedTypes: !11, subprograms: !13, globals: !2, imports: !2)
+!9 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, producer: "clang version 3.5.0 (210479)", isOptimized: false, emissionKind: 1, file: !10, enums: !2, retainedTypes: !11, subprograms: !13, globals: !2, imports: !2)
 !10 = !DIFile(filename: "debug-loc-offset2.cc", directory: "/llvm_cmake_gcc")
 !11 = !{!12}
 !12 = !DICompositeType(tag: DW_TAG_structure_type, name: "A", line: 1, flags: DIFlagFwdDecl, file: !10, identifier: "_ZTS1A")