Avoid unnecessary heap allocations for short-lived strings

[Copilot]

Several hot paths were heap-allocating temporary std::string objects from StringRef/SmallVector sources only to immediately format or compare them.

Changes

• converter_refcount.cpp — getOpcodeStr().str() → std::string_view for binary opcode in ConvertGenericBinaryOperator
• mapper.cpp — build_rust_type — replace std::string(1, sign) + std::to_string(bits) (two allocations + concat) with std::format("{}{}", sign, bits)
• mapper.cpp — ToString unary opcode — replace std::string opcode = getOpcodeStr().str() + string concatenation with std::string_view + std::format
• converter.cpp — enum emitter — eliminate intermediate std::string init_str(init.begin(), init.end()) by passing std::string_view views directly to std::format
• converter.cpp — GetOverloadedFunctionName — std::string name(decl->getNameAsString()) → auto name (removes redundant copy-construction)
• converter.cpp — enum constant formatting — getName().str() → std::string_view(getName())

Example before/after for the enum value emitter:

// Before
std::string init_str(init.begin(), init.end());
StrCat(std::format("{} = {},", e->getNameAsString(), init_str));

// After
StrCat(std::format("{} = {},", std::string_view(e->getName()),
                   std::string_view(init.data(), init.size())));