Ensure that SourceFiles always have a backing buffer in the SourceManager

The "buffer ID" in a SourceFile, which is used to find the source file's
contents in the SourceManager, has always been optional. However, the
effectively every SourceFile actually does have a buffer ID, and the
vast majority of accesses to this information dereference the optional
without checking.

Update the handful of call sites that provided `nullopt` as the buffer
ID to provide a proper buffer instead. These were mostly unit tests
and testing programs, with a few places that passed a never-empty
optional through to the SourceFile constructor.

Then, remove optionality from the representation and accessors. It is
now the case that every SourceFile has a buffer ID, simplying a bunch
of code.

lib/IDE/ModuleInterfacePrinting.cpp

@@ -751,9 +751,7 @@ static SourceLoc getDeclStartPosition(SourceFile &File) {
 }
 
 static void printUntilFirstDeclStarts(SourceFile &File, ASTPrinter &Printer) {
-  if (!File.getBufferID().has_value())
-    return;
-  auto BufferID = *File.getBufferID();
+  auto BufferID = File.getBufferID();
 
   auto &SM = File.getASTContext().SourceMgr;
   CharSourceRange TextRange = SM.getRangeForBuffer(BufferID);