When merging lambdas, keep track of the capture lists from each version.

Different versions of a lambda will in general refer to different
enclosing variable declarations, because we do not merge most
block-scope declarations, such as local variables. Keep track of all the
declarations that correspond to a lambda's capture fields so that we can
rewrite the name of any of those variables to the lambda capture,
regardless of which copy of the body of `operator()` we look at.