Add the ability to pipe trace output to a file. (#1183)

Breaks `--trace` into two flags:

- `--parser_debug`, which sets the parser debug level (which I haven't dug into piping with `--trace`, but seemed likely to be troublesome)
- `--trace_file`, which now the type checker and interpreter will use for trace information (note compile errors should use a different channel)

Most of the file edits are just mechanical testdata flag updates: `sed -i 's/--trace/--parser_debug --trace_file=-/' testdata/**/*.carbon`

To explain the output paths:

- parse/compile errors: stderr
- print() calls: stdout
- parser tracing: `--parser_debug` option, stdout (formerly stdout if `--trace`)
- type check/compile tracing: `--trace_file=<file>`, giving `-` uses stdout (formerly stdout if `--trace`)
- return code of executed Carbon code: `--trace_file` if set, stdout if not (formerly stdout always)

executable_semantics/syntax/parse_and_lex_context.cpp

@@ -15,7 +15,7 @@ auto ParseAndLexContext::RecordSyntaxError(const std::string& message,
   // TODO: support formatting of `SourceLocation` instances with formatv().
   std::string full_message;
   llvm::raw_string_ostream(full_message)
-      << (prefix_with_newline && trace() ? "\n" : "")
+      << (prefix_with_newline && parser_debug() ? "\n" : "")
       << "COMPILATION ERROR: " << source_loc() << ": " << message;
   error_messages_.push_back(full_message);