Make bazel run use the build working dir (#1198)

╚╡bazel run :executable_semantics -- testdata/basic_syntax/print.carbon
INFO: Invocation ID: e7820b06-f118-45c7-afdb-b3a3f526ffcf
INFO: Analyzed target //executable_semantics:executable_semantics (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //executable_semantics:executable_semantics up-to-date:
bazel-bin/executable_semantics/executable_semantics
INFO: Elapsed time: 0.466s, Critical Path: 0.01s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action
INFO: Running command line: bazel-bin/executable_semantics/executable_semantics
INFO: Build completed successfully, 1 total action
Hello world!
result: 0

executable_semantics/BUILD

@@ -35,6 +35,7 @@ cc_binary(
     srcs = ["main_bin.cpp"],
     deps = [
         ":main",
+        "@llvm-project//llvm:Support",
     ],
 )
 

executable_semantics/main_bin.cpp

@@ -3,9 +3,29 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 #include "executable_semantics/main.h"
+#include "llvm/ADT/SmallString.h"
+#include "llvm/Support/FileSystem.h"
+#include "llvm/Support/Path.h"
 
 auto main(int argc, char** argv) -> int {
   // This assumes execution from bazel, in runfiles.
-  return Carbon::ExecutableSemanticsMain(
-      "executable_semantics/data/prelude.carbon", argc, argv);
+  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,
+                          "executable_semantics/data/prelude.carbon");
+
+  // Behave as if the working directory is where `bazel run` was invoked.
+  char* build_working_dir = getenv("BUILD_WORKING_DIRECTORY");
+  if (build_working_dir != nullptr) {
+    if (std::error_code err =
+            llvm::sys::fs::set_current_path(build_working_dir)) {
+      llvm::errs() << "Failed to set working directory: " << err.message();
+      return 1;
+    }
+  }
+
+  return Carbon::ExecutableSemanticsMain(prelude_path, argc, argv);
 }