Improve error message about incompatible function types.

When reporting that a function has illegally been overloaded only
by return type, include "task", "export", and "extern "C"", as appropriate
in the error message to make clear what the issue is.

Finishes issue #216.

module.cpp

@@ -506,10 +506,13 @@ Module::AddFunctionDeclaration(Symbol *funSym, bool isInline) {
                         break;
                 }
                 if (i == functionType->GetNumParameters()) {
+                    std::string thisRetType = functionType->GetReturnTypeString();
+                    std::string otherRetType = ofType->GetReturnTypeString();
                     Error(funSym->pos, "Illegal to overload function by return "
-                          "type only (previous declaration was at line %d of "
-                          "file %s).", overloadFunc->pos.first_line,
-                          overloadFunc->pos.name);
+                          "type only.  This function returns \"%s\" while "
+                          "previous declaration at %s:%d returns \"%s\".",
+                          thisRetType.c_str(), overloadFunc->pos.name,
+                          overloadFunc->pos.first_line, otherRetType.c_str());
                     return;
                 }
             }

type.cpp

@@ -2552,6 +2552,19 @@ FunctionType::GetDIType(llvm::DIDescriptor scope) const {
 }
 
 
+const std::string
+FunctionType::GetReturnTypeString() const {
+    std::string ret;
+    if (isTask)
+        ret += "task ";
+    if (isExported)
+        ret += "export ";
+    if (isExternC)
+        ret += "extern \"C\" ";
+    return ret + returnType->GetString();
+}
+
+
 LLVM_TYPE_CONST llvm::FunctionType *
 FunctionType::LLVMFunctionType(llvm::LLVMContext *ctx, bool includeMask) const {
     if (isTask == true) 

type.h

@@ -776,6 +776,8 @@ public:
 
     const Type *GetReturnType() const { return returnType; }
 
+    const std::string GetReturnTypeString() const;
+
     /** This method returns the LLVM FunctionType that corresponds to this
         function type.  The \c includeMask parameter indicates whether the
         llvm::FunctionType should have a mask as the last argument in its