[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200561 91177308-0d34-0410-b5e6-96231b3b80d8

docs/BitCodeFormat.rst

@@ -746,6 +746,8 @@ function. The operand fields are:
   * ``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