[CMake] Fixing up CMark exports to expand at configuration

Expanding this at include time doesn't work correctly for LLDB because the variable conventions aren't consistent. Expanding and configure time means the Swift variables get expanded and passed along.

Author: Chris Bieneman

cmake/modules/SwiftConfig.cmake.in

@@ -17,8 +17,9 @@ set(SWIFT_LIBRARY_DIR "@SWIFT_CONFIG_LIBRARY_DIRS@")
 set(SWIFT_CMAKE_DIR "@SWIFT_CMAKE_DIR@")
 set(SWIFT_BINARY_DIR "@SWIFT_BINARY_DIR@")
 
-if(NOT TARGET libcmark_static AND ${product}_PATH_TO_CMARK_BUILD)
-  include(${${product}_PATH_TO_CMARK_BUILD}/src/CMarkExports.cmake)
+set(CMARK_TARGETS_FILE @SWIFT_PATH_TO_CMARK_BUILD@/src/CMarkExports.cmake)
+if(NOT TARGET libcmark_static AND EXISTS ${CMARK_TARGETS_FILE})
+  include(${CMARK_TARGETS_FILE})
 endif()
 
 if(NOT TARGET swift)