[lldb][NFC] Another attempt to fix GCC 5.x compilation

37510f69 tried to fix GCC 5.x compilation
by making the enum which is used as a unordered_map key unscoped. However it
seems that in GCC 5.x, enum keys are not supported *at all* in unordered_maps
(at least that's what some trial&error on godbolt tells me). This updates the
workaround to just use an int until GCC 5.x support is dropped.