[SYCL] Fix -Wmain error in dpllvm.cpp

[wenju-he]

reinterpret_cast<void *>(main) is a Clang extension warning (-Wmain) that becomes an error under -Werror. Use the same idom in llvm/tools/llvm-config/llvm-config.cpp.

Fixes a regression from 328217f.

[Fznamznon]

Why don't we do it the same way as llvm tools, instead of adding an empty function, do

/// Compute the path to the main executable.
static std::string getExecutablePath(const char *Argv0) {
  // This just needs to be some symbol in the binary; C++ doesn't
  // allow taking the address of ::main however.
  void *P = (void *)(intptr_t)getExecutablePath;
  return sys::fs::getMainExecutable(Argv0, P);
}

[wenju-he]

Why don't we do it the same way as llvm tools, instead of adding an empty function, do

/// Compute the path to the main executable.
static std::string getExecutablePath(const char *Argv0) {
  // This just needs to be some symbol in the binary; C++ doesn't
  // allow taking the address of ::main however.
  void *P = (void *)(intptr_t)getExecutablePath;
  return sys::fs::getMainExecutable(Argv0, P);
}

this file only has one function: main

[wenju-he]

/// Compute the path to the main executable.
static std::string getExecutablePath(const char *Argv0) {
// This just needs to be some symbol in the binary; C++ doesn't
// allow taking the address of ::main however.
void *P = (void *)(intptr_t)getExecutablePath;
return sys::fs::getMainExecutable(Argv0, P);
}

done, thanks

[wenju-he]

@intel/llvm-gatekeepers please merge, thanks.
BMG fail Test d:\github\_work\llvm\llvm\build-e2e\specconstants\2020\non_native\output\gpu.cpp.tmp.out hung! is reported to #22405