PR16094: I should have known Obj-C init-capture disambiguation couldn't be
*that* easy...

Try a bit harder to disambiguate. This is mostly straightforward, but for
=-style initializers, we actually need to know where an expression ends:

  [foo = bar baz]

is a message send, whereas

  [foo = bar + baz]

is a lambda-introducer. Handle this by parsing the expression eagerly, and
replacing it with an annotation token. By chance, we use the *exact same*
parsing rules in both cases (except that we need to assume we're inside a
message send for the parse, to turn off various forms of inapplicable
error recovery).