fix incorrect error source location on function calling convention

AstGen doesn't seem to report enough information to figure out if a
func decl has a calling convention ast node.
A calling convention could be present without it by marking the
function as inline for example.

Author: Techatrix

src/Zcu.zig

@@ -1280,7 +1280,7 @@ pub const SrcLoc = struct {
                 const node = src_loc.relativeToNodeIndex(node_off);
                 var buf: [1]Ast.Node.Index = undefined;
                 const full = tree.fullFnProto(&buf, node).?;
-                return tree.nodeToSpan(full.ast.callconv_expr);
+                return tree.nodeToSpan(if (full.ast.callconv_expr != 0) full.ast.callconv_expr else node);
             },
 
             .node_offset_fn_type_ret_ty => |node_off| {

test/cases/compile_errors/invalid_variadic_function.zig

@@ -13,5 +13,5 @@ comptime {
 //
 // :1:1: error: variadic function does not support 'auto' calling convention
 // :1:1: note: supported calling conventions: 'x86_64_sysv', 'x86_64_win'
-// :1:1: error: variadic function does not support 'inline' calling convention
-// :1:1: note: supported calling conventions: 'x86_64_sysv', 'x86_64_win'
+// :2:8: error: variadic function does not support 'inline' calling convention
+// :2:8: note: supported calling conventions: 'x86_64_sysv', 'x86_64_win'