[RFC] compiler-rt builtins cleanup and refactoring

With the growing usage of compiler-rt builtins at Google and in projects like Fuchsia, we plan on making several improvements to the builtins codebase; before we start making changes, we would like to first perform a round of cleanups and refactoring to make the later contributions easier.

Concretely, we propose the following:

  • reformat the entire builtins codebase with clang-format
  • change comments to use the uniform C99/C++ single-line style (where appropriate)
  • update comments to use complete English sentences and proper prose

Basically, we are just proposing bringing the builtins code into line with the LLVM coding standards and conventions used across the rest of the project where they apply (clearly, function names have external constraints and so those don’t make sense to change).

Please let us know if you have concerns or other suggestions.