swift234

[CMake] Move swift_llvm_config to SwiftSharedCMakeConfig (for SourceKit).

...and rename it to swift_common_llvm_config.

This is the function that acts like llvm_config but handles the LLVM build
being a different configuration from the Swift (or SourceKit) build, which
can be problematic for multi-configuration build schemes like Xcode.

There's also one fix here: LLVM dependencies for dylibs weren't being
properly added. (This is the "SHARED_LIBRARY" case near the bottom.)

Swift SVN r26631