[InstrProfiling] Don't attempt to create duplicate data variables. (#71998)

Fixes a bug introduced by
commit f95b2f1a ("Reland [InstrProf][compiler-rt] Enable MC/DC
Support in LLVM Source-based Code Coverage (1/3)")

createDataVariable() needs to check that a data variable wasn't already
created before creating it. Previously, this was done inadvertantly in
getOrCreateRegionCounters(), which checked that the RegionCounters was
not created multiple times before creating the counter section and the
data variable. When the creation of the data variable was abstracted
into its own function (createDataVariable()), there was no corresponding
check. This was failing on a case in which an instrumented function was
being inlined into multiple functions and a duplicate data variable was
created, which led to a segfault in emitNameData(). Test case added
based on the repro that also ensures a single data variable was created
in this case.