Merge pull request #3320 from 9rnsr/fix_mangle

[cleanup] Gather `mangleof` property related tests into runnable/mangle.d
Normalize mangling scheme

- Don't encode return type and inferred attributes on parents part.
  * Voldemort type issue is properly solved.
  * The mangled name of function-local symbol is independent from the enclosing function inference.
- Specially mangle lambdas
  * To make independent from the context-ness inference.
  * They have unique name in the enclosing scope, so no conflict will occur.
++s
Handle fwdref and lambda.mangleof
+