[llvm-mc] Better error handling in ENOENT case + test

[davide]

Summary

This is a follow up to r247518, addressing Rafael's post-commit reviews.
As a general note, I think we could do a much better job testing for error conditions in tools. I already anticipated in a previous mail, but while implementing this I noticed that the code coverage we have for this is pretty low. I can arbitrarily remove error checking from various tests and the suite still passes.

[• rafael]

Lgtm! Thanks

llvm/trunk/test/tools/llvm-mc/basic.test

+# RUN: not llvm-mc %t.blah -o %t2 2>&1 | FileCheck --check-prefix=ENOENT %s
+# ENOENT: {{.*}}.blah: {{[Nn]}}o such file or directory

llvm/trunk/tools/llvm-mc/llvm-mc.cpp

int main(int argc, char **argv) {
   const char *ProgName = argv[0];
   const Target *TheTarget = GetTarget(ProgName);
   if (!TheTarget)
     return 1;
   // Now that GetTarget() has (potentially) replaced TripleName, it's safe to
   // construct the Triple object.
   Triple TheTriple(TripleName);
 
-  if (InputFilename != "-" && !sys::fs::exists(InputFilename)) {
-    errs() << ProgName << ": '" << InputFilename << "': "
-           << "No such file\n";
-    return 1;
-  }
   ErrorOr<std::unique_ptr<MemoryBuffer>> BufferPtr =
       MemoryBuffer::getFileOrSTDIN(InputFilename);
   if (std::error_code EC = BufferPtr.getError()) {
-    errs() << ProgName << ": " << EC.message() << '\n';
+    errs() << InputFilename << ": " << EC.message() << '\n';
     return 1;
   }
   MemoryBuffer *Buffer = BufferPtr->get();
 
   SourceMgr SrcMgr;
 
   // Tell SrcMgr about this buffer, which is what the parser will pick up.
   SrcMgr.AddNewSourceBuffer(std::move(*BufferPtr), SMLoc());