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/testdata/basic_syntax/trace.carbon

@@ -2,11 +2,11 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// RUN: %{executable_semantics} --trace %s 2>&1 | \
+// RUN: %{executable_semantics} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 //
 // A lot of output is elided: this is only checking for a few things for simple
-// sanity checking on --trace output.
+// sanity checking on --parser_debug --trace_file=- output.
 //
 // NOAUTOUPDATE
 // CHECK: ********** source program **********