[Builtins] Downgrade duplicate source file warning from a fatal error to a warning.

This is a follow up to r375150 to unbreak the `clang-ppc64be-linux` bot.
The commit caused running the tests to fail due to

```
llvm-lit:
/home/buildbots/ppc64be-clang-multistage-test/clang-ppc64be-multistage/llvm/projects/compiler-rt/test/builtins/Unit/lit.cfg.py:116:
fatal: builtins_source_features contains duplicates:
['librt_has_divtc3']
```

This commit should be reverted once the build system bug for powerpc is
fixed.

llvm-svn: 375162