[modules] Combine Pass.h, PassSupport.h and PassAnalysisSupport.h into one module.

The header files are designed to be used always together (through Pass.h).

Addresses the first part of https://llvm.org/bugs/show_bug.cgi?id=27991

Patch by Cristina Cristescu and me.

Reviewed by Richard Smith.

llvm-svn: 272877