[BOLT] Use getMainExecutable() (#126698)

Use LLVM's getMainExecutable() helper instead of rolling our own. This
will result in standard behavior across platforms, such as making sure
that symlinks are always resolved.

(cherry picked from commit 0abe058)

bolt/tools/driver/llvm-bolt.cpp

@@ -173,24 +173,14 @@ void boltMode(int argc, char **argv) {
   }
 }
 
-static std::string GetExecutablePath(const char *Argv0) {
-  SmallString<256> ExecutablePath(Argv0);
-  // Do a PATH lookup if Argv0 isn't a valid path.
-  if (!llvm::sys::fs::exists(ExecutablePath))
-    if (llvm::ErrorOr<std::string> P =
-            llvm::sys::findProgramByName(ExecutablePath))
-      ExecutablePath = *P;
-  return std::string(ExecutablePath);
-}
-
 int main(int argc, char **argv) {
   // Print a stack trace if we signal out.
   sys::PrintStackTraceOnErrorSignal(argv[0]);
   PrettyStackTraceProgram X(argc, argv);
 
   llvm_shutdown_obj Y; // Call llvm_shutdown() on exit.
 
-  std::string ToolPath = GetExecutablePath(argv[0]);
+  std::string ToolPath = llvm::sys::fs::getMainExecutable(argv[0], nullptr);
 
   // Initialize targets and assembly printers/parsers.
   llvm::InitializeAllTargetInfos();