DebugInfo: Implement the -gsimple-template-names functionality

This excludes certain names that can't be rebuilt from the available
DWARF:

* Atomic types - no DWARF differentiating int from atomic int.
* Vector types - enough DWARF (an attribute on the array type) to do
  this, but I haven't written the extra code to add the attributes
  required for this
* Lambdas - ambiguous with any other unnamed class
* Unnamed classes/enums - would need column info for the type in
  addition to file/line number
* noexcept function types - not encoded in DWARF