Teach CallGraph to look into Generic Lambdas.

Teach CallGraph to look into Generic Lambdas.

CallGraph visited LambdaExpr by getting the Call Operator from
CXXRecordDecl (LambdaExpr::getCallOperator calls
CXXRecordDecl::getLambdaCallOperator), which replaced generic lambda
call operators with the non-instantiated FunctionDecl.  The result was
that the CallGraph would only pick up non-dependent calls.

This patch does a few things:
1- Extend CXXRecordDecl to have a getDependentLambdaCallOperator, which
will get the FunctionTemplateDecl, rather than immediately getting the
TemplateDecl.
2- Define getLambdaCallOperator and getDependentLambdaCallOperator in
terms of a common function.
3- Extend LambdaExpr with a getDependentCallOperator, which just calls
the above function.
4- Changes CallGraph to handle Generic LambdaExprs.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@373247 91177308-0d34-0410-b5e6-96231b3b80d8