[lldb] Fix a off-by-one error in ParseSupportFilesFromPrologue (#71984)

This fixes a subtle and previously harmless off-by-one bug in
ParseSupportFilesFromPrologue. The function accounts for the start index
being one-based for DWARF v4 and earlier and zero-based for DWARF v5 and
later. However, the same care wasn't taken for the end index.

This bug existed unnoticed because GetFileByIndex gracefully handles an
invalid index. However, the bug manifested itself after #71458, which
added a call to getFileNameEntry which requires the index to be valid.

No test as the bug cannot be observed without the changes from #71458.
Once that PR is merged, this will be covered by all the DWARF v5 tests.