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!