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/interpreter/exec_program.h

@@ -10,11 +10,14 @@
 #define EXECUTABLE_SEMANTICS_INTERPRETER_EXEC_PROGRAM_H_
 
 #include "executable_semantics/ast/ast.h"
+#include "llvm/Support/raw_ostream.h"
 
 namespace Carbon {
 
 // Runs the top-level declaration list.
-auto ExecProgram(Nonnull<Arena*> arena, AST ast, bool trace) -> ErrorOr<int>;
+auto ExecProgram(Nonnull<Arena*> arena, AST ast,
+                 std::optional<Nonnull<llvm::raw_ostream*>> trace_stream)
+    -> ErrorOr<int>;
 
 }  // namespace Carbon