Switch to the new MingW ABI.

GCC 4.7 changed the MingW ABI. On the clang side this means that methods now
have the thiscall calling convention by default.

llvm-svn: 197164
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 569d5e8..6b1b88d 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -47,6 +47,11 @@
 Major New Features
 ------------------
 
+- Clang uses the new MingW ABI
+GCC 4.7 changed the mingw ABI. Clang 3.4 and older use the GCC 4.6
+ABI. Clang 3.5 and newer use the GCC 4.7 abi.
+
+
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^