[clang]Interp] Re-add some header deps of Source.h

This is a partial revert of cc69662

The full definition of Decl and Stmt are required by PointerUnion, which
validates the number of free bits in Decl* etc based on type alignment.

Author: sam-mccall

clang/lib/AST/Interp/Source.h

@@ -14,12 +14,12 @@
 #define LLVM_CLANG_AST_INTERP_SOURCE_H
 
 #include "PrimType.h"
+#include "clang/AST/DeclBase.h"
+#include "clang/AST/Stmt.h"
 #include "llvm/ADT/PointerUnion.h"
 #include "llvm/Support/Endian.h"
 
 namespace clang {
-class Stmt;
-class Decl;
 class Expr;
 class SourceLocation;
 class SourceRange;