[clangd][QueryDriver] Dont check for existence of driver

[kadircet]

Summary

Execute implementations already checks for permissions and existence
and returns relevant errors as necessary, so instead of printing our own errors,
we just print theirs.

This also fixes a case in windows where the driver might be missing the .exe
suffix. Previously, clangd would reject such a driver because sys::fs::exists is
strict, whereas the underlying Execute implementation would check with .exe
suffix too.

Fixes https://github.com/clangd/clangd/issues/93

clang-tools-extra/clangd/QueryDriverDatabase.cpp

extractSystemIncludesAndTarget(llvm::SmallString<128> Driver,
   SPAN_ATTACH(Tracer, "driver", Driver);
   SPAN_ATTACH(Tracer, "lang", Lang);
 
   if (!QueryDriverRegex.match(Driver)) {
     vlog("System include extraction: not allowed driver {0}", Driver);
     return llvm::None;
   }
 
-  if (!llvm::sys::fs::exists(Driver)) {
-    elog("System include extraction: {0} does not exist.", Driver);
-    return llvm::None;
-  }
-  if (!llvm::sys::fs::can_execute(Driver)) {
-    elog("System include extraction: {0} is not executable.", Driver);
-    return llvm::None;
-  }
-
   llvm::SmallString<128> StdErrPath;
   if (auto EC = llvm::sys::fs::createTemporaryFile("system-includes", "clangd",
                                                    StdErrPath)) {
     elog("System include extraction: failed to create temporary file with "
          "error {0}",
          EC.message());
     return llvm::None;
   }
if (llvm::any_of(FlagsToPreserve,
         Args.push_back(CommandLine[I]);
         Args.push_back(CommandLine[++I]);
       } else if (Arg.startswith("=")) {
         Args.push_back(CommandLine[I]);
       }
     }
   }
 
+  std::string ErrMsg;
   if (int RC = llvm::sys::ExecuteAndWait(Driver, Args, /*Env=*/llvm::None,
-                                         Redirects)) {
+                                         Redirects, /*SecondsToWait=*/0,
+                                         /*MemoryLimit=*/0, &ErrMsg)) {
     elog("System include extraction: driver execution failed with return code: "
-         "{0}. Args: [{1}]",
-         llvm::to_string(RC), printArgv(Args));
+         "{0} - '{1}'. Args: [{2}]",
+         llvm::to_string(RC), ErrMsg, printArgv(Args));
     return llvm::None;
   }
 
   auto BufOrError = llvm::MemoryBuffer::getFile(StdErrPath);
   if (!BufOrError) {
     elog("System include extraction: failed to read {0} with error {1}",
          StdErrPath, BufOrError.getError().message());
     return llvm::None;