Implement auto return types, removing => returns (#850)

This implements #826, I think covering everything important there.

Regarding ReturnTypeContext, I broke that out because it started feeling like a significant number of args to be passing around, and I think this makes the association inside type checking clearer.

Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/testdata/function/auto_return/add.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn add(x: i32, y: i32) => x + y;
+fn add(x: i32, y: i32) -> auto { return x + y; }
 
 fn main() -> i32 {
   return add(1, 2) - 3;

executable_semantics/testdata/function/auto_return/fail_direct_recurse.carbon

@@ -0,0 +1,25 @@
+// 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
+//
+// RUN: not executable_semantics %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes=false %s
+// RUN: not executable_semantics --trace %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: executable_semantics %s
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/function/auto_return/fail_direct_recurse.carbon:18: could not find `Recurse`
+
+package ExecutableSemanticsTest api;
+
+// This is required to fail even though the Recurse() call's return value isn't
+// used.
+fn Recurse(x: i32, do_recurse: Bool) -> auto {
+  if (do_recurse) {
+    Recurse(x, false);
+  }
+  return x;
+}
+
+fn main() -> i32 {
+  return Recurse(1, true) - 3;
+}

executable_semantics/testdata/function/auto_return/fail_multiple_returns.carbon

@@ -0,0 +1,26 @@
+// 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
+//
+// RUN: not executable_semantics %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes=false %s
+// RUN: not executable_semantics --trace %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: executable_semantics %s
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/function/auto_return/fail_multiple_returns.carbon:18: Only one return is allowed in a function with an `auto` return type.
+
+package ExecutableSemanticsTest api;
+
+fn Add(x: i32, y: i32) -> auto {
+  if (x == 0) {
+    return x;
+  } else if (y == 0) {
+    return y;
+  } else {
+    return x + y;
+  }
+}
+
+fn main() -> i32 {
+  return Add(1, 2) - 3;
+}

executable_semantics/testdata/function/auto_return/fail_no_return.carbon

@@ -0,0 +1,19 @@
+// 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
+//
+// RUN: not executable_semantics %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes=false %s
+// RUN: not executable_semantics --trace %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: executable_semantics %s
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/function/auto_return/fail_no_return.carbon:15: control-flow reaches end of function that provides a `->` return type without reaching a return statement
+
+package ExecutableSemanticsTest api;
+
+fn NoReturn() -> auto {
+}
+
+fn main() {
+  NoReturn();
+}

executable_semantics/testdata/function/auto_return/fail_separate_decl.carbon

@@ -0,0 +1,21 @@
+// 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
+//
+// RUN: not executable_semantics %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes=false %s
+// RUN: not executable_semantics --trace %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: executable_semantics %s
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/function/auto_return/fail_separate_decl.carbon:15: syntax error, unexpected SEMICOLON, expecting LEFT_CURLY_BRACE
+
+package ExecutableSemanticsTest api;
+
+// This declaration is not allowed.
+fn Add(x: i32, y: i32) -> auto;
+
+fn Add(x: i32, y: i32) -> auto { return x + y; }
+
+fn main() -> i32 {
+  return Add(1, 2) - 3;
+}

executable_semantics/testdata/function/auto_return/modify_arg_type.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn Id(t: Type) => t;
+fn Id(t: Type) -> auto { return t; }
 
 // Test non-trivial type expression in parameter type.
 

executable_semantics/testdata/function/auto_return/modify_return_type.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn Id(t: Type) => t;
+fn Id(t: Type) -> auto { return t; }
 
 // Test non-trivial type expression in return type.
 

executable_semantics/testdata/function/auto_return/type.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn Id(t: Type) => t;
+fn Id(t: Type) -> auto { return t; }
 
 // Test non-trivial type expression in variable declaration statement.