RFR: 8293422: DWARF emitted by Clang cannot be parsed

[Christian Hagedorn]

The DWARF debugging symbols emitted by Clang is different from what GCC is emitting. While GCC produces a complete `.debug_aranges` section (which is required in the DWARF parser), Clang does not. As a result, the DWARF parser cannot find the necessary information to proceed and create the line number information:

The `.debug_aranges` section contains address range to compilation unit offset mappings. The parsing algorithm can just walk through all these entries to find the correct address range that contains the library offset of the current pc. This gives us the compilation unit offset into the `.debug_info` section from where we can proceed to parse the line number information.

Without a complete `.debug_aranges` section, we fail with an assertion that we could not find the correct entry. Since [JDK-8293402](https://bugs.openjdk.org/browse/JDK-8293402), we will still get the complete stack trace at least. Nevertheless, we should still fix this assertion failure of course. But that would require a different parsing approach. We need to parse the entire `.debug_info` section instead to get to the correct compilation unit. This, however, would require a lot more work. 

I therefore suggest to disable DWARF parsing for Clang for now and file an RFE to support Clang in the future with a different parsing approach. I'm using the `__clang__` `ifdef` to bail out in `get_source_info()` and disable the `gtests`. I've noticed that we are currently running the `gtests` with `NOT PRODUCT` which I think is not necessary - the gtests should also work fine with product builds. I've corrected this as well but that could also be done separately.

Thanks,
Christian

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Commit messages:
 - 8293422: DWARF emitted by Clang cannot be parsed

Changes: https://git.openjdk.org/jdk/pull/10287/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10287&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293422
  Stats: 7 lines in 2 files changed: 5 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/10287.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10287/head:pull/10287

PR: https://git.openjdk.org/jdk/pull/10287