Add support for embedding pretty printers via the #[debugger_visualizer] attribute. Add tests for embedding pretty printers and update documentation.

Ensure all error checking for `#[debugger_visualizer]` is done up front and not when the `debugger_visualizer` query is run.

Clean up potential ODR violations when embedding pretty printers into the `__rustc_debug_gdb_scripts_section__` section.

Respond to PR comments and update documentation.

compiler/rustc_span/src/lib.rs

@@ -1203,6 +1203,7 @@ impl SourceFileHash {
 #[derive(Copy, PartialEq, PartialOrd, Clone, Ord, Eq, Hash, Debug, Encodable, Decodable)]
 pub enum DebuggerVisualizerType {
     Natvis,
+    GdbPrettyPrinter,
 }
 
 /// A single debugger visualizer file.