[build] Disable cleaning sourcekit-lsp by default, rather than having the flag do nothing

Rather than make this change, swiftlang/sourcekit-lsp@37d003eb7 had the
`--no-clean` flag do nothing, which means the flag can't be used at all.
Instead, switch the flag to `--clean` in swiftlang/sourcekit-lsp#1558 and don't
invoke it by default.

Author: finagolfin

utils/swift_build_support/swift_build_support/products/indexstoredb.py

@@ -83,7 +83,7 @@ def get_dependencies(cls):
 
 
 def run_build_script_helper(action, host_target, product, args,
-                            sanitize_all=False, clean=True):
+                            sanitize_all=False, clean=False):
     script_path = os.path.join(
         product.source_dir, 'Utilities', 'build-script-helper.py')
 
@@ -113,8 +113,8 @@ def run_build_script_helper(action, host_target, product, args,
     elif args.enable_tsan:
         helper_cmd.extend(['--sanitize', 'thread'])
 
-    if not clean:
-        helper_cmd.append('--no-clean')
+    if clean:
+        helper_cmd.append('--clean')
 
     # Pass Cross compile host info unless we're testing.
     # It doesn't make sense to run tests of the cross compile host.

utils/swift_build_support/swift_build_support/products/sourcekitlsp.py

@@ -53,14 +53,14 @@ def should_test(self, host_target):
     def test(self, host_target):
         indexstoredb.run_build_script_helper(
             'test', host_target, self, self.args,
-            self.args.test_sourcekitlsp_sanitize_all, clean=False)
+            self.args.test_sourcekitlsp_sanitize_all)
 
     def should_install(self, host_target):
         return self.args.install_sourcekitlsp
 
     def install(self, host_target):
         indexstoredb.run_build_script_helper(
-            'install', host_target, self, self.args, clean=False)
+            'install', host_target, self, self.args)
 
     @classmethod
     def get_dependencies(cls):