Add support for the Dylan language to ClangASTContext
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.
Reviewed By: clayborg
Subscribers: brucem, lldb-commits, aprantl
Differential Revision: https://reviews.llvm.org/D54886