First pass of semantic analysis for init-captures: check the initializer, build

First pass of semantic analysis for init-captures: check the initializer, build
a FieldDecl from it, and propagate both into the closure type and the
LambdaExpr.

You can't do much useful with them yet -- you can't use them within the body
of the lambda, because we don't have a representation for "the this of the
lambda, not the this of the enclosing context". We also don't have support or a
representation for a nested capture of an init-capture yet, which was intended
to work despite not being allowed by the current standard wording.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181985 91177308-0d34-0410-b5e6-96231b3b80d8