Allow lldb-test to combine -find with -dump-clang-ast

This patch threads an lldb::DescriptionLevel through the typesystem to
allow dumping the full Clang AST (level=verbose) of any lldb::Type in
addition to the human-readable source description (default
level=full). This type dumping interface is currently not exposed
through the SBAPI.

The application is to let lldb-test dump the clang AST of search
results. I need this to test lazy type completion of clang types in
subsequent patches.

Differential Revision: https://reviews.llvm.org/D78329