A LambdaCapture does not have sufficient information
to correctly determine whether it is an init-capture or not.
Doing so requires knowledge held in the LambdaExpr itself.
It the case of a nested capture of an init-capture it is not
sufficient to check (as LambdaCapture::isInitCapture
did) whether the associated VarDecl was from an init-capture.
This patch moves isInitCapture to the LambdaExpr and updates
capture->isInitCapture() to lambda->isInitCapture(capture).