[sourcekitd] Fix a race in EditableTextBuffer::getSnapshot()

Found by TSan!

This was found while trying to reproduce a test failure on a linux bot
while running the test/Misc/stats.swift test.  Hopefully this was the
underlying issue.

rdar://35537968

tools/SourceKit/lib/Support/ImmutableTextBuffer.cpp

@@ -126,6 +126,7 @@ EditableTextBuffer::EditableTextBuffer(StringRef Filename, StringRef Text) {
 }
 
 ImmutableTextSnapshotRef EditableTextBuffer::getSnapshot() const {
+  llvm::sys::ScopedLock L(EditMtx);
   return new ImmutableTextSnapshot(const_cast<EditableTextBuffer*>(this), Root,
                                    CurrUpd);
 }