Revert r158700 and dependent patches r158716, r158717, and r158731.

The original r158700 caused crashes in the gcc test suite,
g++.abi/vtable3a.C among others. It also caused failures in the libc++
test suite.

llvm-svn: 158749
diff --git a/clang/lib/Sema/SemaStmtAttr.cpp b/clang/lib/Sema/SemaStmtAttr.cpp
index 395b9d6..60b2d1e 100644
--- a/clang/lib/Sema/SemaStmtAttr.cpp
+++ b/clang/lib/Sema/SemaStmtAttr.cpp
@@ -48,7 +48,7 @@
 static Attr *ProcessStmtAttribute(Sema &S, Stmt *St, const AttributeList &A,
                                   SourceRange Range) {
   switch (A.getKind()) {
-  case AttributeList::AT_FallThrough:
+  case AttributeList::AT_fallthrough:
     return handleFallThroughAttr(S, St, A, Range);
   default:
     // if we're here, then we parsed an attribute, but didn't recognize it as a