Fix potential constructor ambiguity in BuiltinCallMutator::ValueTypePair (#3268)

BuiltinCallMutator::ValueTypePair defines its own constructor with
arguments (llvm::Value *V, llvm::Type *T)
BuiltinCallMutator::ValueTypePair also uses std::pair constructor with
arguments (llvm::Value *&V, llvm::Type *&T)

This causes an ambiguity that confuses GCC when compiling for C++20.
Issue repro on goldbolt: https://godbolt.org/z/MTnvx6E6T

There are multiple possible fixes. I think the one with the least impact
is to change BuiltinCallMutator::ValueTypePair constructor signature to
conform with std::pair constructor signature.
Fixed version: https://godbolt.org/z/49WzvjK63

Other possibility is to remove the constructors that are redundant. But
maybe they are very important.

Author: jaladreips

lib/SPIRV/SPIRVBuiltinHelper.h

@@ -139,7 +139,6 @@ class BuiltinCallMutator {
              "Must specify a pointer element type if value is a pointer.");
     }
     ValueTypePair(std::pair<llvm::Value *, llvm::Type *> P) : pair(P) {}
-    ValueTypePair(llvm::Value *V, llvm::Type *T) : pair(V, T) {}
     ValueTypePair() = delete;
     using pair::pair;
   };