Allow ada_decode not to decode operators

Allow ada_decode not to decode operators

The new DWARF scanner records names as they appear in DWARF.  However,
because Ada is unusual, it also decodes the Ada names to synthesize
package components for them.  In order for this to work out properly,
gdb also needs a mode where ada_decode can be instructed not to decode
Ada operator names.  That is what this patch implements.