Adapting jsiek's executable semantics tooling for commit. (#237)

Notes versus what jsiek wrote:

- This adopts Bazel for building.
    - System-local versions of bison/flex are used. I found https://github.com/jmillikin/rules_bison, but those print a lot of warnings (things like -Wsign-compare IIRC) which makes builds hard to read. Plus I think the underlying bison_cc_library rule didn't work, so this would really only get a hermetic bison/flex build (helpful, but didn't seem worth more time).
    - I'm adding in a .bazeliskrc to push a somewhat more standard choice of bazel versions. I noticed I was getting unstable versions by default, possible Google-specific, but seemed good to include.
    - The `-lpthread` kludge.
- Turn all of the examples into golden tests.
    - Including adding a golden test rule.
- Fixed various style guide issues. For example:
    - Fixing function names to be CamelCase instead of snake_case (https://google.github.io/styleguide/cppguide.html#Function_Names)
    - Removed exception use (https://google.github.io/styleguide/cppguide.html#Exceptions)
    - File name fixes (https://github.com/carbon-language/carbon-lang/blob/trunk/docs/project/cpp_style_guide.md#file-names)
- Dropped `using` of `std` names -- I believe this is preferred (maybe we should be explicit about this in the Carbon style guide)
- Switched `enum` uses to `enum class` for ease-of-identification.
- Spent some time breaking out files to hopefully be easier to read/edit pieces, and understand relations between structs.
- Added `code requires` to `syntax.ypp` to address include issues

Possibly other things -- but the fundamental structure is, I believe, unchanged. I put in the golden tests pretty early to ensure I wasn't mutating output/results.

executable_semantics/ast/expression.h

@@ -0,0 +1,106 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef EXECUTABLE_SEMANTICS_AST_EXPRESSION_H_
+#define EXECUTABLE_SEMANTICS_AST_EXPRESSION_H_
+
+#include <string>
+#include <vector>
+
+namespace Carbon {
+
+enum class ExpressionKind {
+  AutoT,
+  BoolT,
+  Boolean,
+  Call,
+  FunctionT,
+  GetField,
+  Index,
+  IntT,
+  Integer,
+  PatternVariable,
+  PrimitiveOp,
+  Tuple,
+  TypeT,
+  Variable,
+};
+enum class Operator {
+  Add,
+  And,
+  Eq,
+  Neg,
+  Not,
+  Or,
+  Sub,
+};
+
+struct Expression {
+  int line_num;
+  ExpressionKind tag;
+  union {
+    struct {
+      std::string* name;
+    } variable;
+    struct {
+      Expression* aggregate;
+      std::string* field;
+    } get_field;
+    struct {
+      Expression* aggregate;
+      Expression* offset;
+    } index;
+    struct {
+      std::string* name;
+      Expression* type;
+    } pattern_variable;
+    int integer;
+    bool boolean;
+    struct {
+      std::vector<std::pair<std::string, Expression*>>* fields;
+    } tuple;
+    struct {
+      Operator op;
+      std::vector<Expression*>* arguments;
+    } primitive_op;
+    struct {
+      Expression* function;
+      Expression* argument;
+    } call;
+    struct {
+      Expression* parameter;
+      Expression* return_type;
+    } function_type;
+  } u;
+};
+
+auto MakeVar(int line_num, std::string var) -> Expression*;
+auto MakeVarPat(int line_num, std::string var, Expression* type) -> Expression*;
+auto MakeInt(int line_num, int i) -> Expression*;
+auto MakeBool(int line_num, bool b) -> Expression*;
+auto MakeOp(int line_num, Operator op, std::vector<Expression*>* args)
+    -> Expression*;
+auto MakeUnOp(int line_num, enum Operator op, Expression* arg) -> Expression*;
+auto MakeBinOp(int line_num, enum Operator op, Expression* arg1,
+               Expression* arg2) -> Expression*;
+auto MakeCall(int line_num, Expression* fun, Expression* arg) -> Expression*;
+auto MakeGetField(int line_num, Expression* exp, std::string field)
+    -> Expression*;
+auto MakeTuple(int line_num,
+               std::vector<std::pair<std::string, Expression*>>* args)
+    -> Expression*;
+auto MakeIndex(int line_num, Expression* exp, Expression* i) -> Expression*;
+
+auto MakeTypeType(int line_num) -> Expression*;
+auto MakeIntType(int line_num) -> Expression*;
+auto MakeBoolType(int line_num) -> Expression*;
+auto MakeFunType(int line_num, Expression* param, Expression* ret)
+    -> Expression*;
+auto MakeAutoType(int line_num) -> Expression*;
+
+void PrintExp(Expression* exp);
+
+}  // namespace Carbon
+
+#endif  // EXECUTABLE_SEMANTICS_AST_EXPRESSION_H_