Change CodeGen to use a diagnostic consumer (#5847)

We've been trying to have errors/warnings all go through the diagnostics
consumers instead of straight to stderr.

toolchain/diagnostics/coverage_test.cpp

@@ -43,6 +43,9 @@ constexpr Kind UntestedKinds[] = {
     // This is a little long but is tested in lex/numeric_literal_test.cpp.
     Kind::TooManyDigits,
 
+    // Producing an emit failure may be infeasible.
+    Kind::CodeGenUnableToEmit,
+
     // TODO: This can only fire if the first message in a diagnostic is rooted
     // in a file other than the file being compiled. The language server
     // currently only supports compiling one file at a time. Do one of: