Add support for optional config file includes

Both clang and GCC support including additional config and spec files.

GCC also supports optional file includes (include no error, if missing). But clang does not.

It would be extremely helpful to support optional includes. For me this is to support optional includes that may or may not be present. For example package build system metadata; overrides for different glibc versions - which may or may not be present on a given builder.

Please extend config parsing with a new stanza for an optional file include, that does not error out if the file is missing.

Start by locating LLVM's config parsing code and compare how clang and GCC handle required versus optional file includes. Define the new optional-include stanza and verify that present files are included while missing files do not produce an error; the issue names no specific files or tests.

Written by the indexing model from the issue text.