Add option to install without building.

This allows the build action to be decoupled from the install action, so
packaging systems that expect to be able to do discrete installation
steps post-build can do so cheaply, without having to effectively start
the build over from scratch.

To match existing behavior, this defaults to being disabled. I have not
added this to the macOS branch because I am unable to test the behavior.

This behavior was introduced in other projects that use a similar
build-script-helper pattern: see swiftlang/swift-package-manager#9050,
swiftlang/sourcekit-lsp#2579, swiftlang/swift-format#1171, and
swiftlang/swift-docc#1486.

Author: 3405691582

Utilities/build-script-helper.py

@@ -217,7 +217,8 @@ def handle_invocation(args):
       install(args, args.build_path, targets)
     else:
       bin_path = swiftpm_bin_path(swift_exec, swiftpm_args, env)
-      swiftpm('build', swift_exec, swiftpm_args, env)
+      if not args.install_only:
+        swiftpm('build', swift_exec, swiftpm_args, env)
       non_darwin_install(args, bin_path)
   else:
     assert False, 'unknown action \'{}\''.format(args.action)
@@ -615,6 +616,7 @@ def add_common_args(parser):
     parser.add_argument('--verbose', '-v', action='store_true', help='enable verbose output')
     parser.add_argument('--local_compiler_build', action='store_true', help='driver is being built for use with a local compiler build')
     parser.add_argument('--enable-asan', action='store_true', help='driver is being built with ASAN support')
+    parser.add_argument("--install-only", action="store_true", default=False, help="when true, installs without rebuilding")
 
   subparsers = parser.add_subparsers(title='subcommands', dest='action', metavar='action')
   clean_parser = subparsers.add_parser('clean', help='clean the package')