[MS] Implement __iso_volatile loads/stores as builtins

These are supposed to produce the same as normal volatile
pointer loads/stores. When -volatile:ms is specified,
normal volatile pointers are forced to have atomic semantics
(as is the default on x86 in MSVC mode). In that case,
these builtins should still produce non-atomic volatile
loads/stores without acquire/release semantics, which
the new test verifies.

These are only available on ARM (and on AArch64,
although clang doesn't support AArch64/Windows yet).

This implements what is missing for PR30394, making it possible
to compile C++ for ARM in MSVC mode with MSVC headers.

Differential Revision: https://reviews.llvm.org/D24986

llvm-svn: 282900