Refactor output to be more streaming-focused. (#666)

- Switch code to llvm::raw_ostream as part of standardizing output forms.
    - Preferring llvm::raw_ostream over std::ostream because other tooling code should be expected to rely on llvm more closely, and an overall preference towards library consistency.
    - There are a couple spots in syntax/ that still use std streams, but I'd prefer to take a separate PR to see how best to address those.
    - std::boolalpha doesn't work with llvm, so I've implemented equivalent in a couple places (not enough that it felt like worth making a helper function).
- Implement Print(ostream) as consistently as we can, as an instance member.
    - This facilitates the use of the common/ostream.h template to provide operators.
    - Preferring this approach so that Print is easily accessible via gdb, per suggestion on #executable-semantics.
- Switch code currently calling `type->Print(ostream)` to instead do `ostream << *type`.
- Remove the unused `PrintTypeEnv`, nothing used it and the declaration didn't match the definition.

executable_semantics/ast/expression.h

@@ -10,6 +10,8 @@
 #include <variant>
 #include <vector>
 
+#include "common/ostream.h"
+
 namespace Carbon {
 
 struct Expression;
@@ -176,6 +178,8 @@ struct Expression {
   auto GetCallExpression() const -> const CallExpression&;
   auto GetFunctionTypeLiteral() const -> const FunctionTypeLiteral&;
 
+  void Print(llvm::raw_ostream& out) const;
+
   inline auto tag() const -> ExpressionKind {
     return std::visit([](const auto& t) { return t.Kind; }, value);
   }
@@ -191,8 +195,6 @@ struct Expression {
       value;
 };
 
-void PrintExp(const Expression* exp);
-
 }  // namespace Carbon
 
 #endif  // EXECUTABLE_SEMANTICS_AST_EXPRESSION_H_