[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

fatal: builtins_source_features contains duplicates:

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

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@375162 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/builtins/Unit/lit.cfg.py b/test/builtins/Unit/lit.cfg.py
index 8fdb1a2..d2cd330 100644
--- a/test/builtins/Unit/lit.cfg.py
+++ b/test/builtins/Unit/lit.cfg.py
@@ -111,7 +111,9 @@
 if len(builtins_source_feature_duplicates) > 0:
-  lit_config.fatal(
+  # FIXME(dliew): There's a bug in the build system for powerpc where two definitions of the same
+  # function are present in the library. So just downgrade this to a warning until this is fixed.
+  lit_config.warning(
     'builtins_source_features contains duplicates: {}'.format(