Add '#pragma platina guard "..."' to clang

This commit adds a custom pragma to the clang frontend:
  `#pragma platina guard "..."`
It degrades to the llvm's intrinisc::platina, which stores the
annotation string in the associated metadata node.

Rational is to use that pragma to mark viable, but effectively dead
code paths, e.g. code paths that are only entered during an
initialisation phase.