Import non-const rvalue references as var parameters. (#7125)

When importing a C++ function with an rvalue reference parameter, we
previously produced a Carbon value parameter. This would lead to the
toolchain believing it could pass the address of a non-expiring object
to the function, which would lead to a use-after-move.

Instead, we now map non-const rvalue reference parameters to Carbon
`var` parameters. This forces the object passed into C++ to be unique
and owned by the call. While that's not an exact match for C++ rvalue
reference parameters, given that it provides "always move" not
"conditionally move", it's the closest match we have at the moment.

toolchain/check/pattern_match.cpp

@@ -681,9 +681,9 @@ auto MatchContext::DoVarPreWorkImpl(State state, SemIR::TypeId pattern_type_id,
       auto storage_id = storage_block.AddInstWithCleanup<SemIR::VarStorage>(
           SemIR::LocId(entry.pattern_id),
           {.type_id = pattern_type_id, .pattern_id = entry.pattern_id});
-      // Disable broken lint that suggests a "fix" that doesn't compile.
       auto init_result = Initialize(
           context_, SemIR::LocId(entry.pattern_id),
+          // Disable broken lint that suggests a "fix" that doesn't compile.
           // NOLINTNEXTLINE(performance-move-const-arg)
           std::move(storage_id), std::move(storage_block), scrutinee_id);
       // TODO: Consider instead creating something like a `Temporary`