llvm-build: Factor out duplicate cmake export listing. Patch by Brad King.
llvm-build: Factor out duplicate cmake export listing. Patch by
Brad King.

The write_cmake_fragment and write_cmake_exports_fragment methods share
some logic for selecting libraries that CMake needs to know about.
Factor it out into a helper to avoid duplication.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@245716 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm