De-ASTContext-ify DeclContext.

Remove ASTContext parameter from DeclContext's methods. This change cascaded down to other Decl's methods and changes to call sites started "escalating".
Timings using pre-tokenized "cocoa.h" showed only a ~1% increase in time run between and after this commit.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@74506 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/CGCXX.cpp b/lib/CodeGen/CGCXX.cpp
index 4f9a4ca..5f3acea 100644
--- a/lib/CodeGen/CGCXX.cpp
+++ b/lib/CodeGen/CGCXX.cpp
@@ -323,8 +323,8 @@
   if (RD->getNumBases() > 0)
     return false;
   
-  for (CXXRecordDecl::field_iterator I = RD->field_begin(Context), 
-       E = RD->field_end(Context); I != E; ++I) {
+  for (CXXRecordDecl::field_iterator I = RD->field_begin(), E = RD->field_end();
+         I != E; ++I) {
     // We don't support ctors for fields that aren't POD.
     if (!I->getType()->isPODType())
       return false;