[llvm] Re-use original global name in RelLookupTableConverter (#93626)

Prior, the reltable we create was "reltable." + FuncName which can
result in multiple tables named "reltable." + FuncName + ".{number}" if
we substitute multiple tables in a function. Since we replace the
original global, it makes it easier to just take over the original
global's name. Functionally, this doesn't change the IR emitted, just
global names.

This is a subset of PR 93355 that I'm breaking into multiple patches.