[llvm-link] Improve missing file error message

Add error message showing the missing filename.

Currently, we only get 'No such file or directory' without any(!) further info.
This patch will (upon ENOENT error) create an error which exposes the filename.

Author: mhalk

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

@@ -393,8 +393,16 @@ 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 BufferOrErr = MemoryBuffer::getFileOrSTDIN(File);
+
+    // When we encounter a missing file, make sure we expose its name.
+    if (auto EC = BufferOrErr.getError())
+      if (EC == std::errc::no_such_file_or_directory)
+        ExitOnErr(createStringError(EC, "No such file or directory: '%s'",
+                                    File.c_str()));
+
     std::unique_ptr<MemoryBuffer> Buffer =
-        ExitOnErr(errorOrToExpected(MemoryBuffer::getFileOrSTDIN(File)));
+        ExitOnErr(errorOrToExpected(std::move(BufferOrErr)));
 
     std::unique_ptr<Module> M =
         identify_magic(Buffer->getBuffer()) == file_magic::archive