[build-script] Introduce an option to skip local build.

This is useful for cross-compiling.

<rdar://problem/55916729>

Author: Davide Italiano

utils/build-script-impl

@@ -138,6 +138,7 @@ KNOWN_SETTINGS=(
     skip-build-benchmarks       ""               "set to skip building Swift Benchmark Suite"
     skip-build-external-benchmarks "1"            "set to skip building the external Swift Benchmark Suite. (skipped by default)"
     skip-build-playgroundsupport ""              "set to skip building PlaygroundSupport"
+    skip-local-build            ""               "set to skip building for the current host (useful when crosscompiling)"
     skip-test-cmark             ""               "set to skip testing CommonMark"
     skip-test-lldb              ""               "set to skip testing lldb"
     skip-test-swift             ""               "set to skip testing Swift"
@@ -1049,7 +1050,11 @@ for t in "${CROSS_COMPILE_HOSTS[@]}"; do
     esac
 done
 
-ALL_HOSTS=("${LOCAL_HOST}" "${CROSS_COMPILE_HOSTS[@]}")
+ALL_HOSTS=()
+if [[ ! "${SKIP_LOCAL_BUILD}" ]] ; then
+    ALL_HOSTS=("${ALL_HOSTS[@]}" "${LOCAL_HOST}")
+fi
+ALL_HOSTS=("${ALL_HOSTS[@]}" "${CROSS_COMPILE_HOSTS[@]}")
 
 function has_cross_compile_hosts() {
     if [[ ${#ALL_HOSTS[@]} -gt 1 ]]; then

utils/build_swift/driver_arguments.py

@@ -303,6 +303,10 @@ def create_argument_parser():
                 'device')
     option(['-I', '--ios-all'], store_true('ios_all'),
            help='also build for iOS, and allow all iOS tests')
+
+    option(['--skip-local-build'], toggle_true('skip_local_build'),
+           help='set to skip building for the local platform')
+
     option('--skip-ios', store_false('ios'),
            help='set to skip everything iOS-related')
 

utils/build_swift/tests/expected_options.py

@@ -166,6 +166,7 @@
     'dump_config': False,
     'show_sdks': False,
     'skip_build': False,
+    'skip_local_build': False,
     'stdlib_deployment_targets': None,
     'stress_test': False,
     'swift_analyze_code_coverage': defaults.SWIFT_ANALYZE_CODE_COVERAGE,
@@ -465,6 +466,7 @@ class IgnoreOption(_BaseOption):
     EnableOption('--tsan-libdispatch-test'),
     EnableOption('--long-test'),
     EnableOption('--show-sdks'),
+    EnableOption('--skip-local-build'),
     EnableOption('--stress-test'),
     EnableOption('--test'),
     EnableOption('--test-optimize-for-size'),