[compiler-rt] Fix VisualStudio virtual folders layout

Summary:
This patch is a refactoring of the way cmake 'targets' are grouped.
It won't affect non-UI cmake-generators.

Clang/LLVM are using a structured way to group targets which ease
navigation through Visual Studio UI. The Compiler-RT projects
differ from the way Clang/LLVM are grouping targets.

This patch doesn't contain behavior changes.

Reviewers: kubabrecka, rnk

Subscribers: wang0109, llvm-commits, kubabrecka, chrisha

Differential Revision: http://reviews.llvm.org/D21952

llvm-svn: 275111