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.

llvm-svn: 232622

clang/lib/Frontend/CompilerInvocation.cpp

@@ -2037,7 +2037,7 @@ std::string CompilerInvocation::getModuleHash() const {
     llvm::sys::path::append(systemVersionFile, "SystemVersion.plist");
 
     llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> buffer =
-        llvm::MemoryBuffer::getFile(systemVersionFile.str());
+        llvm::MemoryBuffer::getFile(systemVersionFile);
     if (buffer) {
       code = hash_combine(code, buffer.get()->getBuffer());