The thunk's parameter needs the @in_guaranteed convention if it's a
const reference parameter. However, that convention wasn't being used
because clang importer was removing the const reference from the
type and SILGen was computing the type of the parameter based on the
type without const reference.
This commit fixes the bug by passing the clang function type to
SILDeclRef so that it can be used to compute the correct thunk type.
This fixes a crash when a closure is passed to a C function taking a
pointer to a function that has a const reference struct parameter.
rdar://131321096
Debian 12 ships with a version of libstdc++ that declares a templated constructor of `std::function` with an rvalue-reference parameter. This is not yet supported by Swift.
rdar://125816354
While initializing a `std::function` that takes `const std::string&` as a parameter currently crashes, changing the parameter type to `std::string` should work fine.
Clang is using C++17 standard version by default since Clang 16.
Swift’s ClangImporter should do the same, to make sure that clients who run clang and then swiftc without explicit std version see consistent behavior.
rdar://125777068
This makes sure that `std::function` is imported consistently on supported platforms, and that it allows basic usage: calling a function with `callAsFunction`, initializing an empty function, and passing a function retrieved from C++ back to C++ as a parameter.
rdar://103979602
