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/Sema/SemaDeclAttr.cpp b/lib/Sema/SemaDeclAttr.cpp
index c5274f6..2b71df7 100644
--- a/lib/Sema/SemaDeclAttr.cpp
+++ b/lib/Sema/SemaDeclAttr.cpp
@@ -1326,8 +1326,8 @@
     return;
   }
 
-  RecordDecl::field_iterator Field = RD->field_begin(S.Context),
-                          FieldEnd = RD->field_end(S.Context);
+  RecordDecl::field_iterator Field = RD->field_begin(),
+                          FieldEnd = RD->field_end();
   if (Field == FieldEnd) {
     S.Diag(Attr.getLoc(), diag::warn_transparent_union_attribute_zero_fields);
     return;