[cmake] Add the SDK directory after the toolchain directory when compiling host swift code.

This ensures that we pick up libraries from the toolchain before picking up
libraries from the SDK we are using. Normally it would not matter the order that
we add these -L files since the contents of the SDK and toolchain are
disjoint. Once we begin performing stage2 swift builds though, we no longer have
this property since we pass in the stage1's installed libraries as the SDK
directory and we have not split the two sets of libraries yet. The end result is
that if we have the -L in the previous order, we will pick up just built
compatibility libraries before we pick up the actual compatibility libraries
from the actual toolchain we are using to compile. This results in compilation
breakage.

Author: gottesmm

cmake/modules/AddSwift.cmake

@@ -600,13 +600,22 @@ function(add_swift_host_library name)
     # find all of the necessary swift libraries on Darwin.
     if (NOT ASHL_PURE_SWIFT)
       if (CMAKE_Swift_COMPILER)
-        # Add in the SDK directory for the host platform and add an rpath.
-        target_link_directories(${name} PRIVATE
-          ${SWIFT_SDK_${SWIFT_HOST_VARIANT_SDK}_ARCH_${SWIFT_HOST_VARIANT_ARCH}_PATH}/usr/lib/swift)
         # Add in the toolchain directory so we can grab compatibility libraries
         get_filename_component(TOOLCHAIN_BIN_DIR ${CMAKE_Swift_COMPILER} DIRECTORY)
         get_filename_component(TOOLCHAIN_LIB_DIR "${TOOLCHAIN_BIN_DIR}/../lib/swift/macosx" ABSOLUTE)
         target_link_directories(${name} PUBLIC ${TOOLCHAIN_LIB_DIR})
+
+        # Add in the SDK directory for the host platform.
+        #
+        # NOTE: We do this /after/ target_link_directorying TOOLCHAIN_LIB_DIR to
+        # ensure that we first find libraries from the toolchain, rather than
+        # from the SDK. The reason why this is important is that when we perform
+        # a stage2 build, this path is into the stage1 build. This is not a pure
+        # SDK and also contains compatibility libraries. We need to make sure
+        # that the compiler sees the actual toolchain's compatibility libraries
+        # first before the just built compability libraries or build errors occur.
+        target_link_directories(${name} PRIVATE
+          ${SWIFT_SDK_${SWIFT_HOST_VARIANT_SDK}_ARCH_${SWIFT_HOST_VARIANT_ARCH}_PATH}/usr/lib/swift)
       endif()
     endif()
 
@@ -808,14 +817,23 @@ function(add_swift_host_tool executable)
     # host side tools but link with clang, add the appropriate -L paths so we
     # find all of the necessary swift libraries on Darwin.
     if (CMAKE_Swift_COMPILER)
-      # Add in the SDK directory for the host platform and add an rpath.
-      target_link_directories(${executable} PRIVATE
-        ${SWIFT_SDK_${SWIFT_HOST_VARIANT_SDK}_ARCH_${SWIFT_HOST_VARIANT_ARCH}_PATH}/usr/lib/swift)
       # Add in the toolchain directory so we can grab compatibility libraries
       get_filename_component(TOOLCHAIN_BIN_DIR ${CMAKE_Swift_COMPILER} DIRECTORY)
       get_filename_component(TOOLCHAIN_LIB_DIR "${TOOLCHAIN_BIN_DIR}/../lib/swift/macosx" ABSOLUTE)
       target_link_directories(${executable} PUBLIC ${TOOLCHAIN_LIB_DIR})
 
+      # Add in the SDK directory for the host platform and add an rpath.
+      #
+      # NOTE: We do this /after/ target_link_directorying TOOLCHAIN_LIB_DIR to
+      # ensure that we first find libraries from the toolchain, rather than from
+      # the SDK. The reason why this is important is that when we perform a
+      # stage2 build, this path is into the stage1 build. This is not a pure SDK
+      # and also contains compatibility libraries. We need to make sure that the
+      # compiler sees the actual toolchain's compatibility libraries first
+      # before the just built compability libraries or build errors occur.
+      target_link_directories(${executable} PRIVATE
+        ${SWIFT_SDK_${SWIFT_HOST_VARIANT_SDK}_ARCH_${SWIFT_HOST_VARIANT_ARCH}_PATH}/usr/lib/swift)
+
       if (ASHT_HAS_LIBSWIFT AND SWIFT_TOOLS_ENABLE_LIBSWIFT)
         # Workaround to make lldb happy: we have to explicitly add all libswift modules
         # to the linker command line.