Add explicit (void) cast to unused unique_ptr::release() results

[EricWF]

Summary

This patch adds explicit (void) casts to discarded release() calls to suppress -Wunused-result.

This patch fixes *all* warnings are generated as a result of [applying [[nodiscard]] within libc++](https://reviews.llvm.org/D26596).
Similar fixes were applied to Clang in r286796.

include/llvm/DebugInfo/PDB/IPDBSession.h

public:
   std::unique_ptr<T> getConcreteSymbolById(uint32_t SymbolId) const {
     auto Symbol(getSymbolById(SymbolId));
     if (!Symbol)
       return nullptr;
 
     T *ConcreteSymbol = dyn_cast<T>(Symbol.get());
     if (!ConcreteSymbol)
       return nullptr;
-    Symbol.release();
+    (void)Symbol.release();
     return std::unique_ptr<T>(ConcreteSymbol);
   }
 
   virtual std::unique_ptr<PDBSymbol>
   findSymbolByAddress(uint64_t Address, PDB_SymType Type) const = 0;
 
   virtual std::unique_ptr<IPDBEnumLineNumbers>
   findLineNumbers(const PDBSymbolCompiland &Compiland,

lib/Bitcode/Reader/BitReader.cpp

LLVMBool LLVMGetBitcodeModuleInContext(LLVMContextRef ContextRef,
   void *OldDiagnosticContext = Ctx.getDiagnosticContext();
 
   std::string Message;
   Ctx.setDiagnosticHandler(diagnosticHandler, &Message, true);
   std::unique_ptr<MemoryBuffer> Owner(unwrap(MemBuf));
 
   ErrorOr<std::unique_ptr<Module>> ModuleOrErr =
       getLazyBitcodeModule(std::move(Owner), Ctx);
-  Owner.release();
+  // Release the buffer if we didn't take ownership of it since we never owned
+  // it anyway.
+  (void)Owner.release();
   Ctx.setDiagnosticHandler(OldDiagnosticHandler, OldDiagnosticContext, true);
 
   if (ModuleOrErr.getError()) {
     *OutM = wrap((Module *)nullptr);
     if (OutMessage)
       *OutMessage = strdup(Message.c_str());
     return 1;
   }