Refactor testing exe path and benchmark main handling. (#4216)

Consolidates both main libraries into `//testing/base`, and factors out
the exe path handling for benchmarks and unit tests into a common
library to remove duplication. Refactors how that logic is managed to be
cleaner and avoid a confusing bool that came up in code review.

Updates all the tests and benchmarks that use these. I still need to
update other benchmarks to use the same main, but I wanted to keep this
PR somewhat minimal.

This also fixes a bug noticed in passing that the compilation benchmark
didn't have the required dependency on the benchmark library itself,
just the benchmark main library.

testing/base/benchmark_main.cpp

@@ -0,0 +1,32 @@
+// 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 <benchmark/benchmark.h>
+
+#include "absl/flags/parse.h"
+#include "common/init_llvm.h"
+#include "testing/base/global_exe_path.h"
+
+auto main(int orig_argc, char** orig_argv) -> int {
+  // Do LLVM's initialization first, this will also transform UTF-16 to UTF-8.
+  Carbon::InitLLVM init_llvm(orig_argc, orig_argv);
+
+  Carbon::Testing::SetExePath(orig_argv[0]);
+
+  // Inject a flag to override the defaults for benchmarks. This can still be
+  // disabled by user arguments.
+  llvm::SmallVector<char*> injected_argv_storage(orig_argv,
+                                                 orig_argv + orig_argc + 1);
+  char injected_flag[] = "--benchmark_counters_tabular";
+  injected_argv_storage.insert(injected_argv_storage.begin() + 1,
+                               injected_flag);
+  char** argv = injected_argv_storage.data();
+  int argc = injected_argv_storage.size() - 1;
+
+  benchmark::Initialize(&argc, argv);
+  absl::ParseCommandLine(argc, argv);
+  benchmark::RunSpecifiedBenchmarks();
+  benchmark::Shutdown();
+  return 0;
+}