[Analyzer][VLASizeChecker] Check for VLA size overflow.

Summary:
Variable-length array (VLA) should have a size that fits into
a size_t value. According to the standard: "std::size_t can
store the maximum size of a theoretically possible object of
any type (including array)" (this is applied to C too).

The size expression is evaluated at the definition of the
VLA type even if this is a typedef.
The evaluation of the size expression in itself might cause
problems if it overflows.

Reviewers: Szelethus, baloghadamsoftware, martong, gamesh411

Reviewed By: Szelethus, martong, gamesh411

Subscribers: whisperity, rnkovacs, xazax.hun, baloghadamsoftware, szepet, a.sidorin, mikhail.ramalho, Szelethus, donat.nagy, dkrupp, gamesh411, Charusso, martong, ASDenysPetrov, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79330