Revert "[ms-cxxabi] Add a new calling convention that swaps 'this' and 'sret'"

This reverts commit r200561.

This calling convention was an attempt to match the MSVC C++ ABI for
methods that return structures by value.  This solution didn't scale,
because it would have required splitting every CC available on Windows
into two: one for methods and one for free functions.

Now that we can put sret on the second arg (r208453), and Clang does
that (r208458), revert this hack.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208459 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/CallingConv.h

@@ -137,13 +137,7 @@ namespace CallingConv {
     /// convention differs from the more common \c X86_64_SysV convention
     /// in a number of ways, most notably in that XMM registers used to pass
     /// arguments are shadowed by GPRs, and vice versa.
-    X86_64_Win64 = 79,
-
-    /// \brief The calling convention used for __cdecl methods on win32.
-    /// Differs from the C calling convention only in that the order of the
-    /// first parameter and the sret parameter are swapped.
-    X86_CDeclMethod = 80
-
+    X86_64_Win64 = 79
   };
 } // End CallingConv namespace