C++ PATCH to type_decays_to and lambda in decltype

[Jason Merrill]

lambda return deduction uses type_decays_to, and I noticed that said 
function doesn't completely model the lvalue-to-rvalue conversion: we 
need to drop cv-quals from non-class types.  While trying to write a 
test for this, I noticed that we weren't properly rejecting lambdas in 
unevaluated context.  Since lambdas can't appear in decltype I don't 
think it's actually possible to detect the difference in return type 
deduction in a conforming program, but it's still more correct.

Tested x86_64-unknown-linux-gnu, applied to trunk.
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