Try using getMainExecutable to address argv[0] limitations (#5643)

When finding an executable, this validates that the returned binary is a
symlink back to the same thing as /proc/self/exe, also using that as a
fallback for different things.

Looking back at #3912, we started using `findProgramByName` in order to
avoid path canonicalization done by `GetMainExecutable`. That created
issues as in #5096, wherein an `argv[0]` that's not explicit enough
(`llvm-symbolizer` instead of the full path, done in [LLVM's
Signals.cpp](https://github.com/llvm/llvm-project/blob/4f60f45130c6bd96c79e468fe9927a29af760f56/llvm/lib/Support/Signals.cpp#L198))
leads to incorrect results (finding an `llvm-symbolizer` in `$PATH`).

One option to fix this would be to patch LLVM to provide an absolute
path for `llvm-symbolizer`. However, I'll suggest that passing a
filename in `argv[0]` is not terribly uncommon, and could be a migration
limitation if we force it. The failure mode is also opaque; for example:

```
$ /bin/sh -c "exec -a llvm-symbolizer ./bazel-bin/toolchain/carbon"
error: expected carbon-busybox symlink at `/usr/lib/llvm-19/bin/llvm-symbolizer`
```

Combined with the `setenv` of `LLVM_SYMBOLIZER_PATH` in
`busybox_main.cpp`, this is intended to fix #5096.

common/exe_path_test.cpp

@@ -17,21 +17,28 @@ namespace Carbon {
 namespace {
 
 TEST(ExePath, FailureFallback) {
+  static int static_for_main_addr;
+  std::string running_binary =
+      llvm::sys::fs::getMainExecutable("exe_path_test", &static_for_main_addr);
+
   llvm::SmallString<128> path = llvm::StringRef(getenv("TEST_TMPDIR"));
   llvm::sys::path::append(path, "non_existant_binary");
-  std::string exe_path = FindExecutablePath(path);
-  EXPECT_EQ(path, exe_path);
+  std::string exe_path = FindExecutablePath(path.c_str());
+  EXPECT_EQ(running_binary, exe_path);
 }
 
-TEST(ExePath, File) {
+TEST(ExePath, Symlink) {
+  static int static_for_main_addr;
+  std::string running_binary =
+      llvm::sys::fs::getMainExecutable("exe_path_test", &static_for_main_addr);
+
   llvm::SmallString<128> path = llvm::StringRef(getenv("TEST_TMPDIR"));
   llvm::sys::path::append(path, "test_binary");
-  int fd = -1;
-  std::error_code ec = llvm::sys::fs::openFileForWrite(path, fd);
+  std::error_code ec;
+  std::filesystem::create_symlink(running_binary, path.c_str(), ec);
   ASSERT_TRUE(!ec) << "Error code: " << ec;
-  close(fd);
 
-  std::string exe_path = FindExecutablePath(path);
+  std::string exe_path = FindExecutablePath(path.c_str());
   EXPECT_EQ(path, exe_path);
 }