build-script-impl: propagate --verbose-build to nested CMake builds

The main goal of this change is to ensure that the new build of the
stdlib matches the same level of verbosity of the compiler build that
spawn it.

For now I'm not matching this behaviour to the regular CMake build
products (which would be needed if want to target external projects
configured in LLVM).

Addresses rdar://144256800

Author: edymtt

utils/swift_build_support/swift_build_support/build_script_invocation.py

@@ -552,6 +552,11 @@ def convert_to_impl_arguments(self):
                 impl_env["HOST_VARIABLE_{}__{}".format(
                     host_target.replace("-", "_"), name)] = value
 
+        if args.verbose_build:
+            # This ensures all CMake builds (including the ones
+            # called with `ExternalProject`) have verbose output
+            impl_env["VERBOSE"] = "1"
+
         return (impl_env, impl_args)
 
     def compute_host_specific_variables(self):