[CMake] Bug 19462 - Use the INSTALL(EXPORT ...) to export CMake definitions

Summary: This patch moves all the clang library targets into a ClangTargets export list, and installs it using the CMake install(EXPORT...) command.

Reviewers: rnk

Reviewed By: rnk

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D7623

llvm-svn: 238593