Remove many superfluous SmallString::str() calls.

Now that SmallString is a first-class citizen, most SmallString::str()
calls are not required. This patch removes a whole bunch of them, yet
there are lots more.

There are two use cases where str() is really needed:
1) To use one of StringRef member functions which is not available in
SmallString.
2) To convert to std::string, as StringRef implicitly converts while 
SmallString do not. We may wish to change this, but it may introduce
ambiguity.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232622 91177308-0d34-0410-b5e6-96231b3b80d8

lib/MC/MCDwarf.cpp

@@ -1293,7 +1293,7 @@ const MCSymbol &FrameEmitterImpl::EmitCIE(MCObjectStreamer &streamer,
     Augmentation += "R";
     if (IsSignalFrame)
       Augmentation += "S";
-    streamer.EmitBytes(Augmentation.str());
+    streamer.EmitBytes(Augmentation);
   }
   streamer.EmitIntValue(0, 1);