Virtual modifier explicit mapping is limited to platform-dependent int size

[wismill]

Originally posted by @wismill in #450 (comment)

          @mahkoh Good point, setting `virtual_modifiers` using numeric values is also possible, although this is quite an obscure setting IMHO.

There is a limitation though: the numeric values ExprInteger::ival are int and depending on the target compiler/platform it may not be big enough to hold the mask. Agreed, nowadays on most cases we should have sizeof(int) * CHAR_BIT >= 32, but the C standard guarantee only 16 bits. Such overflow would be completely silent and difficult to debug. Note that overflow is still possible and silent with the current implementation with 32-bits ints: e.g. virtual_modifiers PureVirtual = 0xffffffff + 1;

So we may want to switch ExprInteger::ival from int to the explicit int32_t, which should be plenty enough.