Override checks for modifiers.

test/libsolidity/syntaxTests/inheritance/override/modifier_ambiguous.sol

@@ -0,0 +1,9 @@
+contract A {
+    modifier f() virtual { _; }
+}
+contract B {
+    modifier f() virtual { _; }
+}
+contract C is A, B {
+    modifier f() override(A,B) { _; }
+}

test/libsolidity/syntaxTests/inheritance/override/modifier_ambiguous_fail.sol

@@ -0,0 +1,10 @@
+contract A {
+    modifier f() virtual { _; }
+}
+contract B {
+    modifier f() virtual { _; }
+}
+contract C is A, B {
+}
+// ----
+// THIS NEEDS TO BE AN ERROR

test/libsolidity/syntaxTests/modifiers/illegal_modifier_override.sol

@@ -1,5 +1,6 @@
 contract A { modifier mod(uint a) { _; } }
 contract B is A { modifier mod(uint8 a) { _; } }
 // ----
-// TypeError: (61-89): Overriding modifier is missing 'override' specifier.
 // TypeError: (61-89): Override changes modifier signature.
+// TypeError: (61-89): Overriding modifier is missing 'override' specifier.
+// TypeError: (13-40): Trying to override non-virtual modifier. Did you forget to add "virtual"?

test/libsolidity/syntaxTests/modifiers/legal_modifier_override.sol

@@ -1,2 +1,2 @@
-contract A { modifier mod(uint a) { _; } }
+contract A { modifier mod(uint a) virtual { _; } }
 contract B is A { modifier mod(uint a) override { _; } }

test/libsolidity/syntaxTests/modifiers/non-virtual_modifier_override.sol

@@ -0,0 +1,4 @@
+contract A { modifier mod(uint a) { _; } }
+contract B is A { modifier mod(uint a) override { _; } }
+// ----
+// TypeError: (13-40): Trying to override non-virtual modifier. Did you forget to add "virtual"?