[lldb] Remove default llvm::Triple argument from ClangASTContext constructor

Creating an ASTContext with an unknown triple is rarely a good idea (as usually
all our ASTs have a valid triple that is either from the host or the target) and the
default argument makes it far to easy to implicitly create such an AST. Let's
remove it and force people to pass a triple.

The only place where we don't pass a triple is a DWARFASTParserClangTests
where we now just pass the host triple instead (the test doesn't depend on any
triple so this shouldn't change anything).