per_object_debug_info does not work with Clang 12

[lizan]

Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

Install clang 12 and set CC=clang CXX=clang++
In https://github.com/bazelbuild/examples/tree/main/cpp-tutorial/stage3 run:
bazel build --fission=yes --features=per_object_debug_info //main:hello-world.dwp

Then the output is:

INFO: Analyzed target //main:hello-world.dwp (16 packages loaded, 65 targets configured).
INFO: Found 1 target...
ERROR: /build/examples/cpp-tutorial/stage3/main/BUILD:3:11: output 'main/_objs/hello-greet/hello-greet.pic.dwo' was not created
ERROR: /build/examples/cpp-tutorial/stage3/main/BUILD:3:11: Compiling main/hello-greet.cc failed: not all outputs were created or valid
Target //main:hello-world.dwp failed to build
Use --verbose_failures to see the command lines of failed build steps.
INFO: Elapsed time: 0.238s, Critical Path: 0.09s
INFO: 4 processes: 3 internal, 1 linux-sandbox.
FAILED: Build did NOT complete successfully

What operating system are you running Bazel on?

Ubuntu 18.04

What's the output of bazel info release?

release 4.2.1

Have you found anything relevant by searching the web?

Clang 12 release notes says:

• -gsplit-dwarf no longer implies -g2.

Adding --copt=-g2 can make it work, but bazel should add it with toolchain detection. And it increases binary size when fission is not used.

[keith]

I submitted #14084 for the default crosstools