Only use argv[0] as the main executable name if it exists.

[silvas]

Summary

Under some environments, argv[0] doesn't hold a valid file name, but
sys::fs::getMainExecutable will find the main executable properly.

This patch tweaks the logic to fall back to sys::fs::getMainExecutable
in more situations.

[rsmith]

LGTM

llvm/trunk/lib/Support/Signals.cpp

if (!LLVMSymbolizerPathOrErr)
     LLVMSymbolizerPathOrErr = sys::findProgramByName("llvm-symbolizer");
   if (!LLVMSymbolizerPathOrErr)
     return false;
   const std::string &LLVMSymbolizerPath = *LLVMSymbolizerPathOrErr;
 
   // If we don't know argv0 or the address of main() at this point, try
   // to guess it anyway (it's possible on some platforms).
   std::string MainExecutableName =
-      Argv0.empty() ? sys::fs::getMainExecutable(nullptr, nullptr)
-                    : (std::string)Argv0;
+      sys::fs::exists(Argv0) ? (std::string)Argv0
+                             : sys::fs::getMainExecutable(nullptr, nullptr);
   BumpPtrAllocator Allocator;
   StringSaver StrPool(Allocator);
   std::vector<const char *> Modules(Depth, nullptr);
   std::vector<intptr_t> Offsets(Depth, 0);
   if (!findModulesAndOffsets(StackTrace, Depth, Modules.data(), Offsets.data(),
                              MainExecutableName.c_str(), StrPool))
     return false;
   int InputFD;