llvm: various improvements
llvm: various improvements

* Patch llvm driver to auto detect brew libraries, include files,
  and dynamic loader.
* Remove `pour_bottle?` block, the bottle now should be portable.
* Remove glibc dependency and gcc flags. They should be handled by
  superenv now.
* Remove python dependency. llvm requires python 2 not python 3.
* Add conflicts_with "clang-format" for Linuxbrew.
  In Homebrew, llvm is keg-only, so there is no confliction.
* Fix test.

Closes Linuxbrew/homebrew-core#7565.

Signed-off-by: Michka Popoff <7b0496f66f66ee22a38826c310c38b415671b832@gmail.com>

Linuxbrew/homebrew-core,Linuxbrew/homebrew-core,maxim-belkin/homebrew-core,rwhogg/homebrew-core,LinuxbrewTestBot/homebrew-core,LinuxbrewTestBot/homebrew-core,rwhogg/homebrew-core,maxim-belkin/homebrew-core