typeprettyprinter: fix handling of _ at start of identifiers

This broke e.g. 'AddDefinition' for 'int foo(int const * const _x)'.
resulting in 'foo(const int *const_x)'

Change-Id: I6aae6c4a47e127de109b5fbefdf202ca050a391f
Reviewed-by: Leandro Melo <leandro.melo@nokia.com>
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>