Implement a rudimentary form of generic lambdas.

Specifically, the following features are not included in this commit:
  - any sort of capturing within generic lambdas
  - nested lambdas
  - conversion operator for captureless lambdas
  - ensuring all visitors are generic lambda aware


As an example of what compiles:

template <class F1, class F2>
struct overload : F1, F2 {
    using F1::operator();
    using F2::operator();
    overload(F1 f1, F2 f2) : F1(f1), F2(f2) { }
  };

  auto Recursive = [](auto Self, auto h, auto ... rest) {
    return 1 + Self(Self, rest...);
  };
  auto Base = [](auto Self, auto h) {
      return 1;
  };
  overload<decltype(Base), decltype(Recursive)> O(Base, Recursive);
  int num_params =  O(O, 5, 3, "abc", 3.14, 'a');

Please see attached tests for more examples.

Some implementation notes:

  - Add a new Declarator context => LambdaExprParameterContext to
    clang::Declarator to allow the use of 'auto' in declaring generic
    lambda parameters

  - Augment AutoType's constructor (similar to how variadic
    template-type-parameters ala TemplateTypeParmDecl are implemented) to
    accept an IsParameterPack to encode a generic lambda parameter pack.

  - Add various helpers to CXXRecordDecl to facilitate identifying
    and querying a closure class

  - LambdaScopeInfo (which maintains the current lambda's Sema state)
    was augmented to house the current depth of the template being
    parsed (id est the Parser calls Sema::RecordParsingTemplateParameterDepth)
    so that Sema::ActOnLambdaAutoParameter may use it to create the
    appropriate list of corresponding TemplateTypeParmDecl for each
    auto parameter identified within the generic lambda (also stored
    within the current LambdaScopeInfo).  Additionally,
    a TemplateParameterList data-member was added to hold the invented
    TemplateParameterList AST node which will be much more useful
    once we teach TreeTransform how to transform generic lambdas.

  - SemaLambda.h was added to hold some common lambda utility
    functions (this file is likely to grow ...)

  - Teach Sema::ActOnStartOfFunctionDef to check whether it
    is being called to instantiate a generic lambda's call
    operator, and if so, push an appropriately prepared
    LambdaScopeInfo object on the stack.

  - Teach Sema::ActOnStartOfLambdaDefinition to set the
    return type of a lambda without a trailing return type
    to 'auto' in C++1y mode, and teach the return type
    deduction machinery in SemaStmt.cpp to process either
    C++11 and C++14 lambda's correctly depending on the flag.

  - various tests were added - but much more will be needed.

A greatful thanks to all reviewers including Eli Friedman,
James Dennett and the ever illuminating Richard Smith.  And
yet I am certain that I have allowed unidentified bugs to creep in;
bugs, that I will do my best to slay, once identified!

Thanks!
[lld] Fix unused private fields for fixing build failure on darwin