Replace the broken LambdaCapture::isInitCapture API.

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 LambdaExpr and updates
Capture->isInitCapture() to Lambda->isInitCapture(Capture).

llvm-svn: 236760