PYBIND11_NOINLINE-related cleanup. (#3179)
* Removing pragma for GCC -Wattributes, fixing forward declarations.
* Introducing PYBIND11_NOINLINE_FWD to deal with CUDA, GCC7, GCC8.
* Updating PYBIND11_NOINLINE_DCL in Doxyfile.
* Trying noinline, noinline for {CUDA, GCC7, GCC8}
* Trying noinline, inline for {CUDA, GCC7, GCC8}
* Adding GCC -Wattributes `pragma` in 3 header files.
* Introducing PYBIND11_NOINLINE_GCC_PRAGMA_ATTRIBUTES_NEEDED, used in 9 header files.
* Removing ICC pragma 2196, to see if it is still needed.
* Trying noinline, noinline for ICC
* Trying noinline, inline for ICC
* Restoring ICC pragma 2196, introducing PYBIND11_NOINLINE_FORCED, defined for testing.
* Removing code accidentally left in (was for experimentation only).
* Removing one-time-test define.
* Removing PYBIND11_NOINLINE_FWD macro (after learning that it makes no sense).
* Testing with PYBIND11_NOINLINE_DISABLED. Minor non-functional enhancements.
* Removing #define PYBIND11_NOINLINE_DISABLED (test was successful).
* Removing PYBIND11_NOINLINE_FORCED and enhancing comments for PYBIND11_NOINLINE.
* WIP stripping back
* Making -Wattributes pragma in pybind11 specific to GCC7, GCC8, CUDA.8 files changed
