Ensure builtins use the target default Calling Convention

Ensure builtins use the target default Calling Convention

r355317 changed builtins/allocation functions to use the default calling
convention in order to support platforms that use non-cdecl calling
conventions by default.

However the default calling convention is overridable on Windows 32 bit
implementations with some of the /G options. The intent is to permit the
user to set the calling convention of normal functions, however it
should NOT apply to builtins and C++ allocation functions.

This patch ensures that the builtin/allocation functions always use the
Target specific Calling Convention, ignoring the user overridden version
of said default.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@361507 91177308-0d34-0410-b5e6-96231b3b80d8