[parser] disallow language mode directive in body of function with non-simple parameters
TC39 agreed to disallow "use strict" directives in function body when
non-simple parameter lists are used.
This is a continuation of caitp's CL https://codereview.chromium.org/1281163002/
with some refactorings removed for now.
Still TODO: there is a lot of duplication between the is_simple field of
FormalParametersBase and the NonSimpleParameter property ExpressionClassifier
keeps track of. It should be possible to remove the former with a minor
refactoring of arrow function parsing. This will be attempted in a follow-up CL.