stage2: compile log stores node offset

Previously, compile log stored full SrcLoc info, which included absolute
AST node index. This becomes invalid after an incremental compilation.
To make it survive incremental compilation, store an offset from parent
Decl instead.

src/Module.zig

@@ -64,7 +64,8 @@ import_table: std.StringArrayHashMapUnmanaged(*Scope.File) = .{},
 /// a Decl can have a failed_decls entry but have analysis status of success.
 failed_decls: std.AutoArrayHashMapUnmanaged(*Decl, *ErrorMsg) = .{},
 /// Keep track of one `@compileLog` callsite per owner Decl.
-compile_log_decls: std.AutoArrayHashMapUnmanaged(*Decl, SrcLoc) = .{},
+/// The value is the AST node index offset from the Decl.
+compile_log_decls: std.AutoArrayHashMapUnmanaged(*Decl, i32) = .{},
 /// Using a map here for consistency with the other fields here.
 /// The ErrorMsg memory is owned by the `Scope.File`, using Module's general purpose allocator.
 failed_files: std.AutoArrayHashMapUnmanaged(*Scope.File, ?*ErrorMsg) = .{},
@@ -407,10 +408,14 @@ pub const Decl = struct {
     }
 
     pub fn srcLoc(decl: Decl) SrcLoc {
+        return decl.nodeOffsetSrcLoc(0);
+    }
+
+    pub fn nodeOffsetSrcLoc(decl: Decl, node_offset: i32) SrcLoc {
         return .{
             .file_scope = decl.getFileScope(),
             .parent_decl_node = decl.src_node,
-            .lazy = .{ .node_offset = 0 },
+            .lazy = .{ .node_offset = node_offset },
         };
     }