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/statement.h

@@ -0,0 +1,84 @@
+// 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_STATEMENT_H_
+#define EXECUTABLE_SEMANTICS_AST_STATEMENT_H_
+
+#include <list>
+
+#include "executable_semantics/ast/expression.h"
+
+namespace Carbon {
+
+enum class StatementKind {
+  ExpressionStatement,
+  Assign,
+  VariableDefinition,
+  If,
+  Return,
+  Sequence,
+  Block,
+  While,
+  Break,
+  Continue,
+  Match
+};
+
+struct Statement {
+  int line_num;
+  StatementKind tag;
+  union {
+    Expression* exp;
+    struct {
+      Expression* lhs;
+      Expression* rhs;
+    } assign;
+    struct {
+      Expression* pat;
+      Expression* init;
+    } variable_definition;
+    struct {
+      Expression* cond;
+      Statement* then_stmt;
+      Statement* else_stmt;
+    } if_stmt;
+    Expression* return_stmt;
+    struct {
+      Statement* stmt;
+      Statement* next;
+    } sequence;
+    struct {
+      Statement* stmt;
+    } block;
+    struct {
+      Expression* cond;
+      Statement* body;
+    } while_stmt;
+    struct {
+      Expression* exp;
+      std::list<std::pair<Expression*, Statement*>>* clauses;
+    } match_stmt;
+  } u;
+};
+
+auto MakeExpStmt(int line_num, Expression* exp) -> Statement*;
+auto MakeAssign(int line_num, Expression* lhs, Expression* rhs) -> Statement*;
+auto MakeVarDef(int line_num, Expression* pat, Expression* init) -> Statement*;
+auto MakeIf(int line_num, Expression* cond, Statement* then_stmt,
+            Statement* else_stmt) -> Statement*;
+auto MakeReturn(int line_num, Expression* e) -> Statement*;
+auto MakeSeq(int line_num, Statement* s1, Statement* s2) -> Statement*;
+auto MakeBlock(int line_num, Statement* s) -> Statement*;
+auto MakeWhile(int line_num, Expression* cond, Statement* body) -> Statement*;
+auto MakeBreak(int line_num) -> Statement*;
+auto MakeContinue(int line_num) -> Statement*;
+auto MakeMatch(int line_num, Expression* exp,
+               std::list<std::pair<Expression*, Statement*>>* clauses)
+    -> Statement*;
+
+void PrintStatement(Statement*, int);
+
+}  // namespace Carbon
+
+#endif  // EXECUTABLE_SEMANTICS_AST_STATEMENT_H_