LTO type uniquing: store the Decl field of a DIImportedEntity as a DIRef.

No other functionality changes, DIBuilder testcase is included in a paired
CFE commit.

This relaxes the assertion in isScopeRef to also accept subclasses of
DIScope.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205279 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/DIBuilder.h

@@ -663,7 +663,7 @@ namespace llvm {
     ///             variable
     /// @param Line Line number
     DIImportedEntity createImportedDeclaration(DIScope Context,
-                                               DIDescriptor Decl,
+                                               DIScope Decl,
                                                unsigned Line);
 
     /// insertDeclare - Insert a new llvm.dbg.declare intrinsic call.