[parser] partially revert "use-strict directives in function body affect init block"

[parser] partially revert "use-strict directives in function body affect init block"

TC39 agreed to disallow "use strict" directives in function body when
non-simple parameter lists are used.

This patch reorganizes code to desugar non-simple parameters before the
function body itself is parsed, now that the old requirement is
unnecessary.

Also throws when language mode directives occur in functions with non-simple functions,
and reorganizes a number of tests to make them make sense in the new directive-free
world.

There is a lot of cleanup needed, so initial patch is uploaded just for seeing the
diff clearly.


BUG=
LOG=N
R=rossberg, adamk