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

MSVC always places the 'this' parameter for a method first.  The
implicit 'sret' pointer for methods always comes second.  We already
implement this for __thiscall by putting sret parameters on the stack,
but __cdecl methods require putting both parameters on the stack in
opposite order.

Using a special calling convention allows frontends to keep the sret
parameter first, which avoids breaking lots of assumptions in LLVM and
Clang.

Fixes PR15768 with the corresponding change in Clang.

Reviewers: ributzka, majnemer

Differential Revision: http://llvm-reviews.chandlerc.com/D2663

llvm-svn: 200561
diff --git a/llvm/docs/BitCodeFormat.rst b/llvm/docs/BitCodeFormat.rst
index 38b4010..4c1c94a 100644
--- a/llvm/docs/BitCodeFormat.rst
+++ b/llvm/docs/BitCodeFormat.rst
@@ -746,6 +746,8 @@
   * ``arm_apcscc``: code 66
   * ``arm_aapcscc``: code 67
   * ``arm_aapcs_vfpcc``: code 68
+  * ``x86_thiscallcc``: code 70
+  * ``x86_cdeclmethodcc``: code 80
 
 * isproto*: Non-zero if this entry represents a declaration rather than a
   definition