Fix build with xcode 9.3 (dotnet/corefx#28768)

Fix build with xcode 9.3 (dotnet/corefx#28768)

The c_static_assert definition is causing build problems on OSX.
The compiler complains about the typedef we use to implement the
c_static_assert:
declaration shadows a typedef in the global scope [-Werror,-Wshadow]
This is due to multiple static asserts

To fix it, I have modified the c_static_assert to use _Static_assert
C feature that's available in clang.

Commit migrated from https://github.com/dotnet/corefx/commit/133d190320ecb2ff3f13a4e95c90b8e59bdd75e5