[Macros] Add '-plugin-path' the the toolchain's plugins in Info.plist

So Xcode always prefer the plugins in the toolchain to the SDK's. Since
swift.org toolchains include stdlib, it wants to use the plugins in it.

(cherry picked from commit faa5003)

Author: rintaro

utils/build-script-impl

@@ -3375,6 +3375,7 @@ function build_and_test_installable_package() {
           COMPATIBILITY_VERSION=2
           COMPATIBILITY_VERSION_DISPLAY_STRING="Xcode 8.0"
           DARWIN_TOOLCHAIN_CREATED_DATE="$(date -u +'%a %b %d %T GMT %Y')"
+          TOOLCHAIN_PLUGIN_PATH_DESCRIPTOR='$(TOOLCHAIN_DIR)/usr/lib/swift/host/plugins'
 
           SWIFT_USE_DEVELOPMENT_TOOLCHAIN_RUNTIME="YES"
           if [[ "${DARWIN_TOOLCHAIN_REQUIRE_USE_OS_RUNTIME}" -eq "1" ]]; then
@@ -3409,6 +3410,7 @@ function build_and_test_installable_package() {
           call ${PLISTBUDDY_BIN} -c "Add OverrideBuildSettings:SWIFT_LINK_OBJC_RUNTIME string 'YES'" "${DARWIN_TOOLCHAIN_INFO_PLIST}"
           call ${PLISTBUDDY_BIN} -c "Add OverrideBuildSettings:SWIFT_DEVELOPMENT_TOOLCHAIN string 'YES'" "${DARWIN_TOOLCHAIN_INFO_PLIST}"
           call ${PLISTBUDDY_BIN} -c "Add OverrideBuildSettings:SWIFT_USE_DEVELOPMENT_TOOLCHAIN_RUNTIME string '${SWIFT_USE_DEVELOPMENT_TOOLCHAIN_RUNTIME}'" "${DARWIN_TOOLCHAIN_INFO_PLIST}"
+          call ${PLISTBUDDY_BIN} -c "Add OverrideBuildSettings:OTHER_SWIFT_FLAGS string '\$(inherited) -plugin-path ${TOOLCHAIN_PLUGIN_PATH_DESCRIPTOR}'" "${DARWIN_TOOLCHAIN_INFO_PLIST}"
 
           call chmod a+r "${DARWIN_TOOLCHAIN_INFO_PLIST}"