Change __builtin_sycl_unique_stable_name to just use an Itanium mangling

After significant problems in our downstream with the previous
implementation, the SYCL standard has opted to make using macros/etc to
change kernel-naming-lambdas in any way UB (even passively). As a
result, we are able to just emit the itanium mangling.

However, this DOES require a little work in the CXXABI, as the microsoft
and itanium mangler use different numbering schemes for lambdas.  This
patch adds a pair of mangling contexts that use the normal 'itanium'
mangling strategy to fill in the "DeviceManglingNumber" used previously
by CUDA.

Differential Revision: https://reviews.llvm.org/D110281