c++: constant, array, lambda, template [PR108975]

When a lambda refers to a constant local variable in the enclosing scope, we
tentatively capture it, but if we end up pulling out its constant value, we
go back at the end of the lambda and prune any unneeded captures.  Here
while parsing the template we decided that the dim capture was unneeded,
because we folded it away, but then we brought back the use in the template
trees that try to preserve the source representation with added type info.
So then when we tried to instantiate that use, we couldn't find what it was
trying to use, and crashed.

Fixed by not trying to prune when parsing a template; we'll prune at
instantiation time.

	PR c++/108975

gcc/cp/ChangeLog:

	* lambda.cc (prune_lambda_captures): Don't bother in a template.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/lambda/lambda-const11.C: New test.