Client apps can (and in the case of the MSVC STL do) set cdecl
explicitly on `new` / `delete` implementations. On the other hand, Clang
generates implicit decls with the target's default CC. This is
problematic for SPIR-V, since the default there is spir_function, which
is not compatible. Since we cannot change pre-existing headers /
implementations, this patch sets the CC to C for the implicit host-side
decls, when compiling for device, to prevent the conflict. This is fine
because the host-side overloards do not get emitted on device.
327c64cd6a