swift234

[TypeChecker] Introduce frontend option '-debug-forbid-typecheck-prefix' to protect against unnecessary typechecking,
for testing purposes.

When enabled, if the typechecker tries to typecheck a decl or unresolved identifier with the provided
prefix, then an llvm fatal_error will get triggered.

This approach has the advantage that it is very easy to write tests for unnecessary typechecking for a wide range of functionality,
for the compiler or SourceKit, for code-completion, indexing, etc.

Swift SVN r22003