build-script: fix --skip-build-llvm not having full effect (#82233)

While LLVM cannot be completely disabled when Swift or LLDB are built, we should still allow skipping LLVM configuration and build steps when Swift and LLDB are not built. This can happen, for example, when `build-script` user wants an ad-hoc incremental build with `--skip-build-swift` and `--skip-build-lldb` for products that don't depend on LLVM and Swift, or if LLVM and Swift were already configured and built in previous `build-script` invocations.

Author: MaxDesiatov

utils/swift_build_support/swift_build_support/build_script_invocation.py

@@ -623,7 +623,7 @@ def compute_product_pipelines(self):
         # Swift still needs a few LLVM targets like tblgen to be built for it to be
         # configured. Instead, handle this in the product for now.
         builder.add_product(products.LLVM,
-                            is_enabled=True)
+                            is_enabled=self.args.build_llvm or self.args.build_swift or self.args.build_lldb)
 
         builder.add_product(products.StaticSwiftLinuxConfig,
                             is_enabled=self.args.install_static_linux_config)