[llvm-pdbdump] Add an option to dump full class definitions.

This adds the --class-definitions flag.  If specified, when dumping
types, instead of "class Foo" you will see the full class definition,
with member functions, constructors, access specifiers.

NOTE: Using this option can be very slow, as generating a full class
definition requires accessing many different parts of the PDB.

llvm-svn: 230203