ExpectAndConsume: Diagnose errors automatically

 1) Teach ExpectAndConsume() to emit expected and expected-after diagnostics
    using the generic diagnostic descriptions added in r197972, eliminating another
    set of trivial err_expected_* variations while maintaining existing behaviour.

 2) Lift SkipUntil() recovery out of ExpectAndConsume(). The Expect/Consume
    family of functions are primitive parser operations that now have the
    well-defined property of operating on single tokens. Factoring out recovery
    exposes opportunities for more consistent and tailored error recover at the
    call sites instead of just relying on a bottled SkipUntil formula.

llvm-svn: 198270