[PATCH] [libcxx] Create <__invoke> header that generates the __invoke and __invoke_constexpr functions.

[Eric Fiselier]

Hi mclow.lists, danalbert, jroelofs,

`std::experimental::apply` requires a constexpr implementation of `INVOKE(...)`. Simply marking the current implementation of `__invoke` as constexpr will break existing code due to CWG issue #1581. For this reason we should add a separate function called `__invoke_constexpr` to be used where constexpr is required.

To supply two almost identical `INVOKE(...)` we use the `<__invoke>` header to generate the declarations and definitions by defining the required macros and including the `<__invoke>` header inline. The `<__invoke>` header can be included multiple times.

The macros used are:
* `_LIBCPP_INVOKE_NAME`: The name of the invoke function to be created.
* `_LIBCPP_INVOKE_CONSTEXPR`: The optional `constexpr` decorator for the invoke function.
* `_LIBCPP_DECLARE_INVOKE`: Declare the required invoke function.
* `_LIBCPP_DEFINE_INVOKE`: Define the required invoke function.

I chose not to generate invoke functions using macros because of the amount of code that would have to be within a macro block. It also is likely to yield bad error messages.

This patch also re-enables constexpr support for `std::experimental::apply`.

http://reviews.llvm.org/D9730

Files:
  include/__functional_base
  include/__invoke
  include/experimental/tuple
  include/type_traits
  test/std/experimental/utilities/tuple/tuple.apply/constexpr_types.pass.cpp
  test/std/experimental/utilities/tuple/tuple.apply/ref_qualifiers.pass.cpp

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