[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
1 file changed