[llvm-link] Improve missing file error message

Add error messages showing the missing filenames.

Currently, we only get 'No such file or directory' without any(!) further info.
This patch will (only upon ENOENT error) iterate over all requested files and
print which ones are actually missing.

Author: mhalk

llvm/tools/llvm-link/llvm-link.cpp

@@ -393,8 +393,17 @@ static bool linkFiles(const char *argv0, LLVMContext &Context, Linker &L,
   // Similar to some flags, internalization doesn't apply to the first file.
   bool InternalizeLinkedSymbols = false;
   for (const auto &File : Files) {
+    auto ErrOrExpected = MemoryBuffer::getFileOrSTDIN(File);
+
+    // When we encounter a missing file, print all missing files to stderr.
+    if (auto EC = ErrOrExpected.getError())
+      if (EC == std::errc::no_such_file_or_directory)
+        for (auto &F : Files)
+          if (!llvm::sys::fs::exists(F))
+            errs() << "No such file or directory: '" << F << "'\n";
+
     std::unique_ptr<MemoryBuffer> Buffer =
-        ExitOnErr(errorOrToExpected(MemoryBuffer::getFileOrSTDIN(File)));
+        ExitOnErr(errorOrToExpected(std::move(ErrOrExpected)));
 
     std::unique_ptr<Module> M =
         identify_magic(Buffer->getBuffer()) == file_magic::archive