Applied clang-tidy modernize-use-override globally in order to achieve compliance in current code base.
Added warning `suggest-override` for CMake target `framaCIRGen` in order to detect future breaches.

Paragraph 4.1 (framaCIRGen specific options) of the frama-clang documentation states that the parameters `--info`, `--warn` and `--debug` might be used to enable extra messages at runtime. They don't appear to be implemented right now. Instead, execution gets stuck in an infinite loop.

Adjusted copy of the libc++ implementation in LLVM 11.0:
https://github.com/llvm/llvm-project/blob/release/11.x/libcxx/include/ratio

The explicit cast is necessary even though the value for `t` is passed in as an r-value, because it becomes an l-value as soon as it is assigned to the variable.

This fixes the majority of compile warnings when building with Debian clang version 11.1.0.

WP displays trivial terminates proofs

See merge request frama-c/frama-clang!150

synchronize with frama-c/frama-c!3257

See merge request frama-c/frama-clang!151