[AST][NFC] Silence GCC warning about broken strict aliasing rules

[thopre]

Summary

The deserialize() method would trigger the following warning on GCC <7:

warning: dereferencing type-punned pointer will break
    strict-aliasing rules [-Wstrict-aliasing]

    ParamIdx P(*reinterpret_cast<ParamIdx *>(&S));
                                               ^

&S was previously reinterpret_casted from a ParamIdx into a SerialType,
it is therefore safe to cast back into a ParamIdx. Similar to what was
done in D50608, we replace it with two static_cast via void * which
silences the warning and presumably makes GCC understand that no
strict-aliasing violation is happening.

No functional change intended.

[aaron.ballman]

LGTM, thanks!

clang/include/clang/AST/Attr.h

public:
   /// Produce a representation that can later be passed to \c deserialize to
   /// construct an equivalent \c ParamIdx.
   SerialType serialize() const {
     return *reinterpret_cast<const SerialType *>(this);
   }
 
   /// Construct from a result from \c serialize.
   static ParamIdx deserialize(SerialType S) {
-    ParamIdx P(*reinterpret_cast<ParamIdx *>(&S));
+    // Using this two-step static_cast via void * instead of reinterpret_cast
+    // silences a -Wstrict-aliasing false positive from GCC7 and earlier.
+    void *ParamIdxPtr = static_cast<void *>(&S);
+    ParamIdx P(*static_cast<ParamIdx *>(ParamIdxPtr));
     assert((!P.IsValid || P.Idx >= 1) && "valid Idx must be one-origin");
     return P;
   }
 
   /// Is this parameter index valid?
   bool isValid() const { return IsValid; }
 
   /// Get the parameter index as it would normally be encoded for attributes at