Rearrange and clean up how we disambiguate lambda-introducers from ObjC

message sends, designators, and attributes.

Instead of having the tentative parsing phase sometimes return an
indicator to say what diagnostic to produce if parsing fails and
sometimes ask the caller to run it again, consistently ask the caller to
try parsing again if tentative parsing would fail or is otherwise unable
to completely parse the lambda-introducer without producing an
irreversible semantic effect.

Mostly NFC, but we should recover marginally better in some error cases
(avoiding duplicate diagnostics).

llvm-svn: 361182