stage2: remove destroyed functions from maps

This is likely the cause of the flaky test failures in master branch.
Since we have some test coverage for incremental compilation, it's not
OK to leave proper memory management of Fn objects as "TODO".

Author: andrewrk

src/Module.zig

@@ -84,15 +84,13 @@ string_literal_bytes: std.ArrayListUnmanaged(u8) = .{},
 /// The set of all the generic function instantiations. This is used so that when a generic
 /// function is called twice with the same comptime parameter arguments, both calls dispatch
 /// to the same function.
-/// TODO: remove functions from this set when they are destroyed.
 monomorphed_funcs: MonomorphedFuncsSet = .{},
 /// The set of all comptime function calls that have been cached so that future calls
 /// with the same parameters will get the same return value.
 memoized_calls: MemoizedCallSet = .{},
 /// Contains the values from `@setAlignStack`. A sparse table is used here
 /// instead of a field of `Fn` because usage of `@setAlignStack` is rare, while
 /// functions are many.
-/// TODO: remove functions from this set when they are destroyed.
 align_stack_fns: std.AutoHashMapUnmanaged(*const Fn, SetAlignStack) = .{},
 
 /// We optimize memory usage for a compilation with no compile errors by storing the
@@ -560,6 +558,8 @@ pub const Decl = struct {
             gpa.destroy(extern_fn);
         }
         if (decl.getFunction()) |func| {
+            _ = mod.align_stack_fns.remove(func);
+            _ = mod.monomorphed_funcs.remove(func);
             func.deinit(gpa);
             gpa.destroy(func);
         }
@@ -4094,6 +4094,12 @@ pub fn ensureDeclAnalyzed(mod: *Module, decl_index: Decl.Index) SemaError!void {
             // The exports this Decl performs will be re-discovered, so we remove them here
             // prior to re-analysis.
             mod.deleteDeclExports(decl_index);
+
+            // Similarly, `@setAlignStack` invocations will be re-discovered.
+            if (decl.getFunction()) |func| {
+                _ = mod.align_stack_fns.remove(func);
+            }
+
             // Dependencies will be re-discovered, so we remove them here prior to re-analysis.
             for (decl.dependencies.keys()) |dep_index| {
                 const dep = mod.declPtr(dep_index);