C++ codegen emits definitions where declarations would have been sufficient

[bbannier]

When importing other modules we define full definitions (i.e., function bodies) where declarations would have been sufficient. In fact these definitions are marked inline so we end up having to compile the same body again and again. We do this transitively for all imported modules, so the issue gets worse the longer the import chain and the more types become visible.

# @TEST-EXEC-FAIL: TMPDIR=$PWD spicyc -dj x?.spicy -T # Fail to keep generated files around.

# @TEST-START-FILE x1.spicy
module x1;

public type X1 = unit {
    x1: uint8;
};
# @TEST-END-FILE

# @TEST-START-FILE x2.spicy
module x2;

import x1;

public type X2 = unit {
    x2: uint8;
};
# @TEST-END-FILE

The C++ files emitted for each module contain

• full constructors for every defined type, e.g., in x2_*.cc

  namespace __hlt::x1 {
  inline X1::X1() {
  }
  
  inline X1::X1(std::optional<std::optional<::hilti::rt::integer::safe<uint8_t>>> x1) : X1() {
      if ( x1 ) this->x1 = std::move(*x1);
  }
  }

• full streaming operators for each type, e.g., in x2_*.cc

  static auto __hlt::x1::operator<<(std::ostream& o, const __hlt::x1::X1& x) -> std::ostream& {
    o << ::hilti::rt::to_string(x);
    return o;
  }

• there are also a lot of declarations of types, but these might typically also be found in a header file and seem like a separate issue

It should be possible to drop these definitions for the declarations we already emit.