[analyzer] unix.Malloc: preserve AllocaRegion bound to __builtin_alloca().

Binding __builtin_alloca() return value to the symbolic value kills previous binding to a AllocaRegion established by the core.BuiltinFunctions checker. Other checkers may rely upon this information. Rollback handling of __builtin_alloca() to the way prior to r229850.

llvm-svn: 231160