[CMake] Add paths for building Swift code against host toolchain
When building Swift code into the compiler (e.g, the new Swift parser
along with macros support), make sure we always add the appropriate
paths to (1) link against the host Swift toolchain, and (2) find the
host Swift libraries (such as SwiftSyntax) at runtime.
The CMake code for doing this was only running for Darwin builds, so
generalize it to also work on Linux.
(cherry picked from commit 277619a604ee415206abe9c8d87f836c8e9b816a)
