[-Wunsafe-buffer-usage] Safe-buffers re-architecture to introduce Fixable gadgets

Re-architecture of safe-buffers gadgets to re-classify them as warning and fixable
gadgets. The warning gadgets identify unsafe operations on buffer variables and
emit suitable warnings. While the fixable gadgets consider all operations on
variables identified by the warning gadgets and emit necessary fixits.

Differential Revision: https://reviews.llvm.org/D140062?id=486625