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

utils/TableGen/FixedLenDecoderEmitter.cpp

@@ -1091,7 +1091,7 @@ unsigned FilterChooser::getDecoderIndex(DecoderSet &Decoders,
   // overkill for now, though.
 
   // Make sure the predicate is in the table.
-  Decoders.insert(Decoder.str());
+  Decoders.insert(StringRef(Decoder));
   // Now figure out the index for when we write out the table.
   DecoderSet::const_iterator P = std::find(Decoders.begin(),
                                            Decoders.end(),