[Sema] add -Walloca to flag uses of alloca

This CL adds an optional warning to diagnose uses of the
__builtin_alloca family of functions. The use of these functions is
discouraged by many, so it seems like a good idea to allow clang to warn
about it.

Patch by Elaina Guan!

Differential Revision: https://reviews.llvm.org/D64883