[TableGen] Include <set> (NFC)

This patch adds "#include <set>" to several files that are relying on
transitive includes of <set>.  It in turn unblocks the removal of
unnecessary includes of llvm/ADT/SmallSet.h in several other files.

Author: kazutakahirata

llvm/utils/TableGen/CallingConvEmitter.cpp

@@ -16,6 +16,7 @@
 #include "llvm/TableGen/Record.h"
 #include "llvm/TableGen/TableGenBackend.h"
 #include <deque>
+#include <set>
 
 using namespace llvm;
 

llvm/utils/TableGen/FastISelEmitter.cpp

@@ -26,6 +26,7 @@
 #include "llvm/TableGen/Error.h"
 #include "llvm/TableGen/Record.h"
 #include "llvm/TableGen/TableGenBackend.h"
+#include <set>
 #include <utility>
 using namespace llvm;
 

llvm/utils/TableGen/X86FoldTablesEmitter.cpp

@@ -19,6 +19,7 @@
 #include "llvm/Support/X86FoldTablesUtils.h"
 #include "llvm/TableGen/Record.h"
 #include "llvm/TableGen/TableGenBackend.h"
+#include <set>
 
 using namespace llvm;
 using namespace X86Disassembler;