cmake: Add CLANG_LINK_CLANG_DYLIB option

Summary:
Setting CLANG_LINK_CLANG_DYLIB=ON causes clang tools to link against
libclang_shared.so instead of the individual component libraries.

Reviewers: mgorny, beanz, smeenai, phosek, sylvestre.ledru

Subscribers: arphaman, cfe-commits, llvm-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63503

llvm-svn: 365092