[build-script] Install 'earlyswiftsyntax' if 'install-swift' is off

Some build scenario (e.g. lldb) want swift-syntax libraries installed
but don't want to install "swift" compiler products.

Install 'earlyswiftsyntax' products independently when
'--install-swiftsyntax' is specified but '--install-swift' is NOT
specified.

Author: rintaro

utils/swift_build_support/swift_build_support/products/earlyswiftsyntax.py

@@ -73,10 +73,10 @@ def test(self, host_target):
         pass
 
     def should_install(self, host_target):
-        # The artifacts are copied to build directory of 'swift' and are
-        # installed as a part of 'swift' product.
-        return False
+        # When '--install-swift' is enabled, earlyswiftsyntax libraries are installed
+        # from 'swift' product.
+        return (self.should_build(host_target) and self.args.install_swiftsyntax and
+                "--install-swift" not in self.args.build_script_impl_args)
 
     def install(self, host_target):
-        # No-op.
-        pass
+        self.install_with_cmake(["install"], self.host_install_destdir(host_target))