If a variant part has a 128-bit discriminator, then
DwarfUnit::constructTypeDIE will assert. This patch fixes the problem
by allowing any size of integer to be used here. This is largely
accomplished by moving part of DwarfUnit::addConstantValue to a new
method.
Fixes#119655
(cherry picked from commit 3c2807624d2006fa8aacf9c6441c9a3034a52b44)