[compiler-rt][builtins] Add compiler flags to catch potential errors

that can lead to security vulnerabilities

Also, fix a few places that were causing -Wshadow and
-Wformat-nonliteral warnings to be emitted.

This reapplies the patch that was reverted in 0d66dc57 because it
broke a few bots.

I made changes so that cmake checks whether some of the flags are
supported by the compiler that is used before adding them to the list.
Also, I moved function add_security_warnings to CompilerRTUtils.cmake so
that it is defined before it's used.

Differential Revision: https://reviews.llvm.org/D131714