Add string parsing and a print builtin (#721)

It was in my mind to add String in order to support libraries in `package`.  `print` is added in order to have a String go to stdout. I've tried to do `print` in a way that won't be too hard to add other printable types, but it's probably also somewhat optional here -- that is, if desired, I could remove it. But it was a lot easier to doublecheck `\n` behavior with it, and I suspect it'll be helpful in other tests if it supports more value types.

On the side, this also fixes dereferencing in Pattern/Expression Print() calls, which I was noticing printing pointers instead of values. This may be another argument for moving away from passing pointers, since this seems to be a difficult-to-catch error.

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

executable_semantics/testdata/print1.carbon

@@ -0,0 +1,9 @@
+// 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
+
+fn main() -> i32 {
+  var s: auto = "Hello world!\n";
+  Print(s);
+  return 0;
+}

executable_semantics/testdata/print1.golden

@@ -0,0 +1,2 @@
+Hello world!
+result: 0

executable_semantics/testdata/string1.carbon

@@ -0,0 +1,14 @@
+// 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
+
+fn CompareStr(s: String) -> i32 {
+  if (s == "str") {
+    return 0;
+  }
+  return 1;
+}
+
+fn main() -> i32 {
+  return CompareStr("str");
+}

executable_semantics/testdata/string1.golden

@@ -0,0 +1 @@
+result: 0

executable_semantics/testdata/string2.carbon

@@ -0,0 +1,14 @@
+// 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
+
+fn CompareStr(s: String) -> i32 {
+  if (s == "str\n") {
+    return 0;
+  }
+  return 1;
+}
+
+fn main() -> i32 {
+  return CompareStr("str\n");
+}

executable_semantics/testdata/string2.golden

@@ -0,0 +1 @@
+result: 0

executable_semantics/testdata/string3.carbon

@@ -0,0 +1,14 @@
+// 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
+
+fn CompareStr(s: String) -> i32 {
+  if (s == "str;") {
+    return 0;
+  }
+  return 1;
+}
+
+fn main() -> i32 {
+  return CompareStr("\x73\x74\x72\x3B");
+}

executable_semantics/testdata/string3.golden

@@ -0,0 +1 @@
+result: 0

executable_semantics/testdata/string_fail1.carbon

@@ -0,0 +1,8 @@
+// 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
+
+fn main() -> i32 {
+  Print("str\e");
+  return 0;
+}

executable_semantics/testdata/string_fail1.golden

@@ -0,0 +1,2 @@
+COMPILATION ERROR: 6: Invalid escaping in string: "str\e"
+EXIT CODE: 255

executable_semantics/testdata/string_fail2.carbon

@@ -0,0 +1,8 @@
+// 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
+
+fn main() -> i32 {
+  Print("str\x");
+  return 0;
+}

executable_semantics/testdata/string_fail2.golden

@@ -0,0 +1,2 @@
+COMPILATION ERROR: 6: Invalid escaping in string: "str\x"
+EXIT CODE: 255

executable_semantics/testdata/string_fail3.carbon

@@ -0,0 +1,8 @@
+// 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
+
+fn main() -> i32 {
+  Print("str\xaa");
+  return 0;
+}

executable_semantics/testdata/string_fail3.golden

@@ -0,0 +1,2 @@
+COMPILATION ERROR: 6: Invalid escaping in string: "str\xaa"
+EXIT CODE: 255

executable_semantics/testdata/string_fail4.carbon

@@ -0,0 +1,8 @@
+// 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
+
+fn main() -> i32 {
+  Print("str\01");
+  return 0;
+}

executable_semantics/testdata/string_fail4.golden

@@ -0,0 +1,2 @@
+COMPILATION ERROR: 6: Invalid escaping in string: "str\01"
+EXIT CODE: 255

executable_semantics/testdata/string_fail5.carbon

@@ -0,0 +1,9 @@
+// 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
+
+fn main() -> i32 {
+  Print("new
+line");
+  return 0;
+}

executable_semantics/testdata/string_fail5.golden

@@ -0,0 +1,2 @@
+COMPILATION ERROR: 6: invalid character '\x22' in source file.
+EXIT CODE: 255