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.h

@@ -15,14 +15,14 @@ namespace Carbon {
 
 // Returns the AST representing the contents of the named file, or an error code
 // if parsing fails. Allocations go into the provided arena.
-auto Parse(Nonnull<Arena*> arena, std::string_view input_file_name, bool trace)
-    -> ErrorOr<Carbon::AST>;
+auto Parse(Nonnull<Arena*> arena, std::string_view input_file_name,
+           bool parser_debug) -> ErrorOr<Carbon::AST>;
 
 // Equivalent to `Parse`, but parses the contents of `file_contents`.
 // `input_file_name` is used only for reporting source locations, and does
 // not need to name a real file.
 auto ParseFromString(Nonnull<Arena*> arena, std::string_view input_file_name,
-                     std::string_view file_contents, bool trace)
+                     std::string_view file_contents, bool parser_debug)
     -> ErrorOr<Carbon::AST>;
 
 }  // namespace Carbon