[CMake] Use variables exported by SwiftConfig to compose LLDB variables.
[CMake] Use variables exported by SwiftConfig to compose LLDB variables.

The SwiftConfig.cmake file exports variables that point to the swift
binary and library directory. Use those instead of going through the
LLVM_LIBRARY_OUTPUT_INTDIR indirection, which might be incorrect, for
example for out-of-tree builds with the Xcode generator.

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb