[ELF] Lookup INPUT argument in the current directory

[atanasyan]

Summary

If an argument of the INPUT directive is a regular path, linker should lookup it in the current folder first.

The fix does not contain any test cases because I think it is not a good idea to pollute a current folder (which in general might be arbitrary) by test files.

[ruiu]

LGTM with a nit.

ELF/LinkerScript.cpp

182–185

} else if (sys::fs::exists(S)) {

ELF/LinkerScript.cpp

if (sys::path::is_absolute(S)) {
     Driver->addFile(S);
   } else if (S.startswith("=")) {
     if (Config->Sysroot.empty())
       Driver->addFile(S.substr(1));
     else
       Driver->addFile(Saver.save(Config->Sysroot + "/" + S.substr(1)));
   } else if (S.startswith("-l")) {
     Driver->addFile(searchLibrary(S.substr(2)));
   } else {
+    if (sys::fs::exists(S))
+      Driver->addFile(S);
+    else {
       std::string Path = findFromSearchPaths(S);
       if (Path.empty())
         error("Unable to find " + S);
       Driver->addFile(Saver.save(Path));
     }
   }
+}
 
 void LinkerScript::readAsNeeded() {
   expect("(");
   bool Orig = Config->AsNeeded;
   Config->AsNeeded = true;
   for (;;) {
     StringRef Tok = next();
     if (Tok == ")")