Fixes ICE when overriding an implemented modifier with an unimplemented one.

test/libsolidity/syntaxTests/modifiers/modifier_abstract_override.sol

@@ -0,0 +1,11 @@
+contract A {
+    modifier m() virtual { _; }
+}
+abstract contract B is A {
+    modifier m() virtual override;
+}
+contract C is B {
+    function f() m public {}
+}
+// ----
+// TypeError 4593: (78-108): Overriding an implemented modifier with an unimplemented modifier is not allowed.

test/libsolidity/syntaxTests/modifiers/multiple_inheritance_unimplemented_override.sol

@@ -0,0 +1,11 @@
+contract A {
+    modifier m() virtual { _; }
+}
+abstract contract B {
+    modifier m() virtual;
+}
+contract C is A, B {
+    modifier m() override(A, B) { _; }
+    function f() m public {}
+}
+// ----

test/libsolidity/syntaxTests/modifiers/unimplemented_override_unimplemented.sol

@@ -0,0 +1,11 @@
+abstract contract A {
+    modifier m() virtual;
+}
+abstract contract B is A {
+    modifier m() virtual override;
+}
+abstract contract C is B {
+    modifier m() virtual override;
+    function f() m public {}
+}
+// ----

test/libsolidity/syntaxTests/modifiers/use_unimplemented_from_base.sol

@@ -0,0 +1,8 @@
+abstract contract A {
+    modifier m() virtual;
+    function f() m public {}
+}
+contract B is A {
+    modifier m() virtual override { _; }
+}
+// ----

test/libsolidity/syntaxTests/modifiers/use_unimplemented_on_overridden_func.sol

@@ -0,0 +1,8 @@
+abstract contract A {
+    modifier m() virtual;
+    function f() m public virtual {}
+}
+abstract contract B is A {
+    function f() public override {}
+}
+// ----