Allow GoogleTest tests to locate their executable path. (#3992)

This also factors out the code for doing this location from the driver
to a tiny helper library.

The motivation for this is letting tests locate data files like the
prelude or other files needed by the toolchain more easily. A subsequent
PR will use this heavily in the Clang runner and related logic for
example.

common/exe_path.cpp

@@ -0,0 +1,23 @@
+// 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/exe_path.h"
+
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/FileSystem.h"
+#include "llvm/Support/Program.h"
+
+namespace Carbon {
+
+auto FindExecutablePath(llvm::StringRef argv0) -> std::string {
+  if (!llvm::sys::fs::exists(argv0)) {
+    if (llvm::ErrorOr<std::string> path = llvm::sys::findProgramByName(argv0)) {
+      return std::move(*path);
+    }
+  }
+
+  return argv0.str();
+}
+
+}  // namespace Carbon