Improve the specification of spellings in Attr.td.

Note that this is mostly a structural patch that handles the change from the old
spelling style to the new one. One consequence of this is that all AT_foo_bar
enum values have changed to not be based off of the first spelling, but rather
off of the class name, so they are now AT_FooBar and the like (a straw poll on
IRC showed support for this). Apologies for code churn.

Most attributes have GNU spellings as a temporary solution until everything else
is sorted out (such as a Keyword spelling, which I intend to add if someone else
doesn't beat me to it). This is definitely a WIP.

I've also killed BaseCheckAttr since it was unused, and I had to go through
every attribute anyway.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158700 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Sema/DeclSpec.cpp b/lib/Sema/DeclSpec.cpp
index f6764c2..c57699b 100644
--- a/lib/Sema/DeclSpec.cpp
+++ b/lib/Sema/DeclSpec.cpp
@@ -753,7 +753,7 @@
   writtenBS.ModeAttr = false;
   AttributeList* attrs = getAttributes().getList();
   while (attrs) {
-    if (attrs->getKind() == AttributeList::AT_mode) {
+    if (attrs->getKind() == AttributeList::AT_Mode) {
       writtenBS.ModeAttr = true;
       break;
     }