[NFC][IR] Avoid string leak with target extension types.

[nikic]

LG

[hctim]

Testing as we speak :)

[hctim]

Succeeds with this patch, fails without. Looks good to me.

llvm/include/llvm/IR/DerivedTypes.h

 ///
 /// Target extension types have a string name, and optionally have type and/or
 /// integer parameters. The exact meaning of any parameters is dependent on the
 /// target.
 class TargetExtType : public Type {
   TargetExtType(LLVMContext &C, StringRef Name, ArrayRef<Type *> Types,
                 ArrayRef<unsigned> Ints);
 
-  std::string Name;
+  // These strings are ultimately owned by the context.
+  StringRef Name;
   unsigned *IntParams;
 
 public:
   TargetExtType(const TargetExtType &) = delete;
   TargetExtType &operator=(const TargetExtType &) = delete;
 
   /// Return a target extension type having the specified name and optional
   /// type and integer parameters.

llvm/lib/IR/Type.cpp

 }
 
 //===----------------------------------------------------------------------===//
 //                       TargetExtType Implementation
 //===----------------------------------------------------------------------===//
 
 TargetExtType::TargetExtType(LLVMContext &C, StringRef Name,
                              ArrayRef<Type *> Types, ArrayRef<unsigned> Ints)
-    : Type(C, TargetExtTyID), Name(Name) {
+    : Type(C, TargetExtTyID), Name(C.pImpl->Saver.save(Name)) {
   NumContainedTys = Types.size();
 
   // Parameter storage immediately follows the class in allocation.
   Type **Params = reinterpret_cast<Type **>(this + 1);
   ContainedTys = Params;
   for (Type *T : Types)
     *Params++ = T;