Bug 1604952 - Rename FunctionCreationData.h to Stencil.h r=tcampbell

Because I'm planning on moving non-function data into this, I think the time is
potentially right to start calling this stencil.h

Differential Revision: https://phabricator.services.mozilla.com/D57902

--HG--
rename : js/src/frontend/FunctionCreationData.h => js/src/frontend/Stencil.h
extra : moz-landing-system : lando