Export witness table and RTTI objects in compiled libraries. (#1514)

* Export witness table objects in compiled code.

- Ensure that witness tables are preceeded with `extern "C"` modifier in the generated C++ code.
- RTTI objects use the mangled name of the type directly, so that can be queried using the type's mangled name directly from the resulting DLL.
- Expose `Linkage::getTypeConformanceWitnessMangledName` to return the mangled name of witness tables to the host.
- Ensure that all witness tables (including those for associated types) have proper mangled name.

* Fix GCC error in Slang generated code.

1 files changed, 8 insertions, 2 deletions

diff --git a/source/slang/slang-emit-cpp.cpp b/source/slang/slang-emit-cpp.cpp
index 0c2b4cd93..e7c90f657 100644
--- a/source/slang/slang-emit-cpp.cpp
+++ b/source/slang/slang-emit-cpp.cpp
@@ -1611,7 +1611,7 @@ void CPPSourceEmitter::emitWitnessTable(IRWitnessTable* witnessTable)
     _maybeEmitWitnessTableTypeDefinition(interfaceType);
 
     // Define a global variable for the witness table.
-    m_writer->emit("extern ");
+    m_writer->emit("extern \"C\" ");
     emitSimpleType(interfaceType);
     m_writer->emit(" ");
     m_writer->emit(getName(witnessTable));
@@ -1692,7 +1692,13 @@ void CPPSourceEmitter::emitInterface(IRInterfaceType* interfaceType)
 
 void CPPSourceEmitter::emitRTTIObject(IRRTTIObject* rttiObject)
 {
-    m_writer->emit("static TypeInfo ");
+    // Declare the type info object as `extern "C"` first.
+    m_writer->emit("extern \"C\" TypeInfo ");
+    m_writer->emit(getName(rttiObject));
+    m_writer->emit(";\n");
+
+    // Now actually define the object.
+    m_writer->emit("TypeInfo ");
     m_writer->emit(getName(rttiObject));
     m_writer->emit(" = {");
     auto typeSizeDecoration = rttiObject->findDecoration<IRRTTITypeSizeDecoration>();