[llvm-link] Improve missing file error message (llvm#82514)

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.

Change-Id: I2a240e66576980e6878891e10ad3c5972b47ea1f

Author: mhalk

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

@@ -386,8 +386,16 @@ 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