[cmake] Create exports for umbrella library targets
[cmake] Create exports for umbrella library targets

When using the umbrella llvm-libraries and clang-libraries targets, we
should export all library targets, otherwise they'll be part of our
distribution but not usable from the CMake package.

Differential Revision: https://reviews.llvm.org/D58862

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@355354 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang