Allow explorer to be run directly from the bazel build directory. (#2257)

Previously explorer worked under `bazel run` but not when run directly, because it was finding the prelude relative to the current working directory. Now it looks for it relative to the location of the binary, so it works both when run directly and when run with `bazel run`.

explorer/main_bin.cpp

@@ -8,13 +8,15 @@
 #include "llvm/Support/Path.h"
 
 auto main(int argc, char** argv) -> int {
-  // This assumes execution from bazel, in runfiles.
-  llvm::SmallString<256> prelude_path;
-  if (std::error_code err = llvm::sys::fs::current_path(prelude_path)) {
-    llvm::errs() << "Failed to get working directory: " << err.message();
-    return 1;
-  }
-  llvm::sys::path::append(prelude_path, "explorer/data/prelude.carbon");
+  // This assumes execution from `bazel-bin/explorer`, either directly or with
+  // `bazel run`.
+  static int static_for_main_addr;
+  std::string exe = llvm::sys::fs::getMainExecutable(
+      argv[0], static_cast<void*>(&static_for_main_addr));
+  llvm::SmallString<256> prelude_path = llvm::sys::path::parent_path(exe);
+  llvm::sys::path::append(prelude_path, "explorer.runfiles", "carbon",
+                          "explorer", "data");
+  llvm::sys::path::append(prelude_path, "prelude.carbon");
 
   // Behave as if the working directory is where `bazel run` was invoked.
   char* build_working_dir = getenv("BUILD_WORKING_DIRECTORY");