[V2][PATCH 0/1]Add a new warning option -Wstrict-flex-arrays

[Qing Zhao via Gcc-patches]

Hi, this is the 2nd version for this patch.

Per our discussion, I made the following change compared to the first
version:

1. The level of -Warray-bounds will NOT control how a trailing array 
   is considered as a flex array member anymore. Only the level of  
   -fstrict-flex-arrays will control this;
2. Updating the documentation for -Warray-bounds by clarifying this
   change.
3. Updating the testing cases for such change.

I have bootstrapped and regression tested on both X86 and aarch64
without any issue.

Okay for committing?

thanks.

Qing