* make it always return a fully qualified name. stage1 is inconsistent
   about this.
 * AstGen: fix anon_name_strategy to correctly be `func` when anon type
   creation happens in the operand of the return expression.
 * Sema: implement type names for the "function" naming strategy.
 * Put "enum", "union", "opaque", or "struct" in place of "anon" when
   creating respective anonymous Decl names.
 * std.testing: add `expectStringStartsWith`. Didn't end up using it
   after all.

Also this enables the real test runner for stage2 LLVM backend (sans
wasm32) since it works now.