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/2009-11-03-InsertExtractValue.ll b/llvm/test/DebugInfo/2009-11-03-InsertExtractValue.ll
index f8382af..c992a43 100644
--- a/llvm/test/DebugInfo/2009-11-03-InsertExtractValue.ll
+++ b/llvm/test/DebugInfo/2009-11-03-InsertExtractValue.ll
@@ -9,7 +9,7 @@
 !2 = !DISubroutineType(types: !3)
 !3 = !{null}
 !4 = !DIFile(filename: "/foo", directory: "bar.cpp")
-!5 = !DICompileUnit(language: DW_LANG_C99, isOptimized: true, emissionKind: 0, file: !4, enums: !{}, retainedTypes: !{})
+!5 = distinct !DICompileUnit(language: DW_LANG_C99, isOptimized: true, emissionKind: 0, file: !4, enums: !{}, retainedTypes: !{})
 
 define <{i32, i32}> @f1() {
 ; CHECK: !dbgx ![[NUMBER:[0-9]+]]