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>

common/string_helpers.cpp

@@ -0,0 +1,88 @@
+// 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
+
+#include "common/string_helpers.h"
+
+#include "common/check.h"
+#include "llvm/ADT/StringExtras.h"
+
+namespace Carbon {
+
+// Carbon only takes uppercase hex input.
+static auto FromHex(char c) -> std::optional<char> {
+  if (c >= '0' && c <= '9') {
+    return c - '0';
+  }
+  if (c >= 'A' && c <= 'F') {
+    return 10 + c - 'A';
+  }
+  return std::nullopt;
+}
+
+auto UnescapeStringLiteral(llvm::StringRef source)
+    -> std::optional<std::string> {
+  std::string ret;
+  ret.reserve(source.size());
+  size_t i = 0;
+  while (i < source.size()) {
+    char c = source[i];
+    if (c == '\\') {
+      ++i;
+      if (i == source.size()) {
+        return std::nullopt;
+      }
+      switch (source[i]) {
+        case 'n':
+          ret.push_back('\n');
+          break;
+        case 'r':
+          ret.push_back('\r');
+          break;
+        case 't':
+          ret.push_back('\t');
+          break;
+        case '0':
+          if (i + 1 < source.size() && llvm::isDigit(source[i + 1])) {
+            // \0[0-9] is reserved.
+            return std::nullopt;
+          }
+          ret.push_back('\0');
+          break;
+        case '"':
+          ret.push_back('"');
+          break;
+        case '\'':
+          ret.push_back('\'');
+          break;
+        case '\\':
+          ret.push_back('\\');
+          break;
+        case 'x': {
+          i += 2;
+          if (i >= source.size()) {
+            return std::nullopt;
+          }
+          std::optional<char> c1 = FromHex(source[i - 1]);
+          std::optional<char> c2 = FromHex(source[i]);
+          if (c1 == std::nullopt || c2 == std::nullopt) {
+            return std::nullopt;
+          }
+          ret.push_back(16 * *c1 + *c2);
+          break;
+        }
+        case 'u':
+          FATAL() << "\\u is not yet supported in string literals";
+        default:
+          // Unsupported.
+          return std::nullopt;
+      }
+    } else {
+      ret.push_back(c);
+    }
+    ++i;
+  }
+  return ret;
+}
+
+}  // namespace Carbon