[Sema] Add warning for unused lambda captures

Summary:
Warn when a lambda explicitly captures something that is not used in its body.

The warning is part of -Wunused and can be enabled with -Wunuse

[Sema] Add warning for unused lambda captures

Summary:
Warn when a lambda explicitly captures something that is not used in its body.

The warning is part of -Wunused and can be enabled with -Wunused-lambda-capture.

Reviewers: rsmith, arphaman, jbcoe, aaron.ballman

Subscribers: Quuxplusone, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D28467

llvm-svn: 291905

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Improve our handling of lambda expressions that occur within default
arguments. There are two aspects to this:

- Make sure that when marking the declarations referenced in a
default argument, we

Improve our handling of lambda expressions that occur within default
arguments. There are two aspects to this:

- Make sure that when marking the declarations referenced in a
default argument, we don't try to mark local variables, both because
it's a waste of time and because the semantics are wrong: we're not
in a place where we could capture these variables again even if it
did make sense.
- When a lambda expression occurs in a default argument of a
function template, make sure that the corresponding closure type is
considered dependent, so that it will get properly instantiated. The
second bit is a bit of a hack; to fix it properly, we may have to
rearchitect our handling of default arguments, parsing them only
after creating the function definition. However, I'd like to
separate that work from the lambdas work.

llvm-svn: 151076

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Implement C++11 [expr.lambda.prim]p13, which prohibits lambdas in
default arguments if in fact those lambdas capture any entity.

llvm-svn: 150282