Add an implementation of C11's stdatomic.h

Adds a Clang-specific implementation of C11's stdatomic.h header. On systems,
such as FreeBSD, where a stdatomic.h header is already provided, we defer to
that header instead (using our __has_include_next technology). Otherwise, we
provide an implementation in terms of our __c11_atomic_* intrinsics (that were
created for this purpose).

C11 7.1.4p1 requires function declarations for atomic_thread_fence,
atomic_signal_fence, atomic_flag_test_and_set,
atomic_flag_test_and_set_explicit, and atomic_flag_clear, and requires that
they have external linkage. Accordingly, we provide these declarations, but if
a user elides the shadowing macros and uses them, then they must have a libc
(or similar) that actually provides definitions.

atomic_flag is implemented using _Bool as the underlying type. This is
consistent with the implementation provided by FreeBSD and also GCC 4.9 (at
least when __GCC_ATOMIC_TEST_AND_SET_TRUEVAL == 1).

Patch by Richard Smith (rebased and slightly edited by me -- Richard said I
should drive at this point).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218957 91177308-0d34-0410-b5e6-96231b3b80d8
Revert useless part of r217349

Adding handling of __builtin_assume_aligned to IntExprEvaluator does not make
sense because __builtin_assume_aligned returns a pointer (not an integer).
Thanks to Richard for figuring out why this was not doing anything.

I'll add this back in a better place (PointerExprEvaluator perhaps).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218958 91177308-0d34-0410-b5e6-96231b3b80d8
Make test/Sema/atomic-ops.c free-standing

This test includes stdint.h, which might include system headers (and that might
not work, depending on the system configuration). Attempting to fix
llvm-clang-lld-x86_64-debian-fast.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218959 91177308-0d34-0410-b5e6-96231b3b80d8