[Solaris] replace Solaris.h hack with a set of better hacks

Summary:
Got rid of unwieldy -include Solaris.h portability solution, replacing it with interposed header and moving endian defines into Host.h.

Fixes PR28370.

Reviewers: joerg, alekseyshl, mgorny

Reviewed By: joerg

Subscribers: llvm-commits, mgorny, ro, krytarowski

Patch by Fedor Sergeev.

Differential Revision: https://reviews.llvm.org/D3413

llvm-svn: 306002