[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.

Author: DougGregor

cmake/modules/AddSwift.cmake

@@ -576,7 +576,7 @@ function(_add_swift_runtime_link_flags target relpath_to_lib_dir bootstrapping)
     # Make sure we can find the early SwiftSyntax libraries.
     target_link_directories(${target} PRIVATE "${SWIFT_PATH_TO_EARLYSWIFTSYNTAX_BUILD_DIR}/lib/swift/host")
 
-    # For the "end step" of bootstrapping configurations on Darwin, need to be
+    # For the "end step" of bootstrapping configurations, we need to be
     # able to fall back to the SDK directory for libswiftCore et al.
     if (BOOTSTRAPPING_MODE MATCHES "BOOTSTRAPPING.*")
       if (NOT "${bootstrapping}" STREQUAL "1")
@@ -590,6 +590,13 @@ function(_add_swift_runtime_link_flags target relpath_to_lib_dir bootstrapping)
           get_filename_component(TOOLCHAIN_BIN_DIR ${SWIFT_EXEC_FOR_SWIFT_MODULES} DIRECTORY)
           get_filename_component(TOOLCHAIN_LIB_DIR "${TOOLCHAIN_BIN_DIR}/../lib/swift/${SWIFT_SDK_${SWIFT_HOST_VARIANT_SDK}_LIB_SUBDIR}" ABSOLUTE)
           target_link_directories(${target} PUBLIC ${TOOLCHAIN_LIB_DIR})
+        else()
+          get_filename_component(swift_bin_dir ${SWIFT_EXEC_FOR_SWIFT_MODULES} DIRECTORY)
+          get_filename_component(swift_dir ${swift_bin_dir} DIRECTORY)
+          set(host_lib_dir "${swift_dir}/lib/swift/${SWIFT_SDK_${SWIFT_HOST_VARIANT_SDK}_LIB_SUBDIR}")
+          target_link_directories(${target} PRIVATE ${host_lib_dir})
+
+          set(swift_runtime_rpath "${host_lib_dir}")
         endif()
       endif()
     endif()
@@ -926,10 +933,17 @@ function(add_swift_host_tool executable)
       endif()
     endif()
 
-    set_property(
-      TARGET ${executable}
-      APPEND PROPERTY INSTALL_RPATH
-        "@executable_path/../${extra_relative_rpath}lib/swift/host")
+    if(${SWIFT_HOST_VARIANT_SDK} IN_LIST SWIFT_DARWIN_PLATFORMS)
+      set_property(
+        TARGET ${executable}
+        APPEND PROPERTY INSTALL_RPATH
+          "@executable_path/../${extra_relative_rpath}lib/swift/host")
+    else()
+      set_property(
+        TARGET ${executable}
+        APPEND PROPERTY INSTALL_RPATH
+          "$ORIGIN/../${extra_relative_rpath}lib/swift/host")
+    endif()
   endif()
 
   if(ASHT_THINLTO_LD64_ADD_FLTO_CODEGEN_ONLY)