[lldb] [ObjectFileMachO] LLVM_COV is not mapped into firmware memory (#86359)

It is possible to gather code coverage in a firmware environment, where
the __LLVM_COV segment will not be mapped in memory but does exist in
the binary, see

https://llvm.org/devmtg/2020-09/slides/PhippsAlan_EmbeddedCodeCoverage_LLVM_Conf_Talk_final.pdf

The __LLVM_COV segment in the binary happens to be at the same address
as the __DATA segment, so if lldb treats this segment as loaded, it
shadows the __DATA segment and address->symbol resolution can fail.

For these non-userland code cases, we need to mark __LLVM_COV as not a
loadable segment.

rdar://124475661