Various cleanups to definitions in the runtime

[hughbe]

@gparker42

Commit 1:

• Only defines SWIFT_USE_SWIFTCALL if we actually can use SWIFTCALL.
• Also clean up now redundant s390x check

Commit 2:

• Don't use these attributes if we don't have a compiler that supports them
• This increases the portability of the shims, and helps resolve some Visual Studio contribution experience nits

Commit 3:

• Don't bother working around Darwin module being pulled in by stdint.h on Windows, for obvious reasons

[gparker42]

Is _WIN32 the right check here? I was expecting a check for clang versus MSVC.

[gparker42]

Alternatively, if the __INT_TYPE__ versions are a Darwin-specific workaround then they should be behind an affirmative check for Darwin. Then everyone else defaults to the cleaner stdint.h form.

[hughbe]

BTW, we have to define these here, as these are installed headers - we don't want to rely on external LLVM files here.

[hughbe]

Here is the very odd Linux error..

/usr/include/x86_64-linux-gnu/sys/types.h:146:10: error: 'stddef.h' file not found
#include <stddef.h>
         ^
<module-includes>:1:10: note: in file included from <module-includes>:1:
#include "AssertionReporting.h"

Looks like a bug?
http://askubuntu.com/questions/390805/clang-error-stddef-h-file-not-found
https://bugs.launchpad.net/ubuntu/+source/llvm-defaults/+bug/1242300
https://bugs.launchpad.net/ubuntu/+source/llvm-defaults/+bug/1198123
http://stackoverflow.com/questions/20587228/clang-error-stddef-file-not-found
android/ndk#215

[hughbe]

@swift-ci please smoke test@swift-ci please smoke test

[gparker42]

Looks good.