[build] Add back intermediate pipeline so that the corelibs are not built before the compiler tests are run

This reverts 0132841 but moves libdispatch into the earlier pipeline, so that
it alone is built before the compiler validation suite is run.

Author: finagolfin

utils/swift_build_support/swift_build_support/build_script_invocation.py

@@ -603,7 +603,7 @@ def compute_product_pipelines(self):
         # the build-script code base. The main difference is that these are all
         # build, tested, and installed all at once instead of performing build,
         # test, install like a normal build-script product.
-        builder.begin_impl_pipeline(should_run_epilogue_operations=True)
+        builder.begin_impl_pipeline(should_run_epilogue_operations=False)
 
         builder.add_impl_product(products.LibCXX,
                                  is_enabled=self.args.build_libcxx)
@@ -615,6 +615,11 @@ def compute_product_pipelines(self):
                                  is_enabled=self.args.build_lldb)
         builder.add_impl_product(products.LibDispatch,
                                  is_enabled=self.args.build_libdispatch)
+
+        # Begin a new build-script-impl pipeline that builds libraries that we
+        # build as part of build-script-impl but that we should eventually move
+        # onto build-script products.
+        builder.begin_impl_pipeline(should_run_epilogue_operations=True)
         builder.add_impl_product(products.Foundation,
                                  is_enabled=self.args.build_foundation)
         builder.add_impl_product(products.XCTest,