Make the non-const DeclContext::getParent call the const version, instead of the other way around.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@59646 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/AST/DeclBase.cpp b/lib/AST/DeclBase.cpp
index 7e51ef5..88d9a20 100644
--- a/lib/AST/DeclBase.cpp
+++ b/lib/AST/DeclBase.cpp
@@ -353,10 +353,10 @@
 // DeclContext Implementation
 //===----------------------------------------------------------------------===//
 
-DeclContext *DeclContext::getParent() {
-  if (ScopedDecl *SD = dyn_cast<ScopedDecl>(this))
+const DeclContext *DeclContext::getParent() const {
+  if (const ScopedDecl *SD = dyn_cast<ScopedDecl>(this))
     return SD->getDeclContext();
-  else if (BlockDecl *BD = dyn_cast<BlockDecl>(this))
+  else if (const BlockDecl *BD = dyn_cast<BlockDecl>(this))
     return BD->getParentContext();
   else
     return NULL;