Added a new class, ASTDumper, that provides verbose

diagnostics of Clang AST classes for the purpose of
debugging the types LLDB produces for DWARF objects.

The ASTDumper is currently only used in log output
if you enable verbose mode in the expression log:

log enable -v lldb expr

Its output then appears in the log for external
variables used by the expr command.

llvm-svn: 124703