[Support] Use C++11 attribute syntax for visibility attributes

The gnu extension __attribute syntax cannot be mixed with the
C++11 alignas specifier, so in order to use visibility attributes on
classes that also use alignas, we need to use the C++11 standard syntax.

Also fix a few warnings introduced by this change.

Reviewed By: compnerd

Differential Revision: https://reviews.llvm.org/D152043