Emit warning if response file is not found

[sepavloff]

Summary

If construct @foo is specified in command line, CommandLine library
tries to consider foo as a name of response file and read options
from it. If file foo is not found, it tries to open file @foo.
This behavior is compatible with libiberty, used in GCC and other
utilities.

On the other hand, if user specifies @foo, it almost certainly
means a response file named foo. If foo was not found, it is
some error like misspelling or wrong current directory but not a file
with name @foo. With this change CommandLine issues warning
response file 'foo' not found and continues execution as before.

Construct @file is a possible way to load configuration files
(see https://reviews.llvm.org/D24933). If it will be accepted,
CommandLibrary will not search for file @file, instead it will
try to load configuration file file.cfg. This warning could help
in identifying compatibility problems.

lib/Support/CommandLine.cpp

for (unsigned I = 0; I != Argv.size();) {
     // contents.  Nested response files are expanded in subsequent iterations.
     SmallVector<const char *, 0> ExpandedArgv;
     if (!ExpandResponseFile(Arg + 1, Saver, Tokenizer, ExpandedArgv,
                             MarkEOLs, RelativeNames)) {
       // We couldn't read this file, so we leave it in the argument stream and
       // move on.
       AllExpanded = false;
       ++I;
+      // Emit warning if response file is not found, it might be misprint.
+      errs() << "Response file '" << Arg + 1 << "' not found\n";
       continue;
     }
     Argv.erase(Argv.begin() + I);
     Argv.insert(Argv.begin() + I, ExpandedArgv.begin(), ExpandedArgv.end());
   }
   return AllExpanded;
 }