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/empty-array.ll b/llvm/test/DebugInfo/X86/empty-array.ll
index ce460a7..bad080c 100644
--- a/llvm/test/DebugInfo/X86/empty-array.ll
+++ b/llvm/test/DebugInfo/X86/empty-array.ll
@@ -27,7 +27,7 @@
 !llvm.dbg.cu = !{!0}
 !llvm.module.flags = !{!21}
 
-!0 = !DICompileUnit(language: DW_LANG_C_plus_plus, producer: "clang version 3.3 (trunk 169136)", isOptimized: false, emissionKind: 0, file: !20, enums: !1, retainedTypes: !1, subprograms: !1, globals: !3, imports:  !1)
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, producer: "clang version 3.3 (trunk 169136)", isOptimized: false, emissionKind: 0, file: !20, enums: !1, retainedTypes: !1, subprograms: !1, globals: !3, imports:  !1)
 !1 = !{}
 !3 = !{!5}
 !5 = !DIGlobalVariable(name: "a", line: 1, isLocal: false, isDefinition: true, scope: null, file: !6, type: !7, variable: %class.A* @a)