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.

llvm-svn: 74506
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 9d76592..4a9c420 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -242,8 +242,8 @@
 
   // FIXME: This is linear time.
   unsigned i = 0;
-  for (RecordDecl::field_iterator Field = RD->field_begin(Info.Ctx),
-                               FieldEnd = RD->field_end(Info.Ctx);
+  for (RecordDecl::field_iterator Field = RD->field_begin(),
+                               FieldEnd = RD->field_end();
        Field != FieldEnd; (void)++Field, ++i) {
     if (*Field == FD)
       break;