Delete Host/windows/win32.h

It's always hard to remember when to include this file, and
when you do include it it's hard to remember what preprocessor
check it needs to be behind, and then you further have to remember
whether it's windows.h or win32.h which you need to include.

This patch changes the name to PosixApi.h, which is more appropriately
named, and makes it independent of any preprocessor setting.

There's still the issue of people not knowing when to include this,
because there's not a well-defined set of things it exposes other
than "whatever is missing on Windows", but at least this should
make it less painful to fix when problems arise.

This patch depends on LLVM revision r278170.

llvm-svn: 278177