Add support for the Dylan language to ClangASTContext

Summary:
This change adds eLanguageTypeDylan to the set of languages supported
by ClangASTContext. Debug info generated by the Open Dylan compiler's
LLVM back-end was designed to be compatible with C debug info.

Patch by Peter Housel.

Reviewers: clayborg

Reviewed By: clayborg

Subscribers: brucem, lldb-commits, aprantl

Differential Revision: https://reviews.llvm.org/D54886