Scanner: Generate error on inbalanced RLO/LRO/PDF override markers.

test/libsolidity/syntaxTests/literals/unicode_string_direction_override_1.sol

@@ -0,0 +1,9 @@
+contract C {
+    function f() public pure
+    {
+        // PDF
+        bytes memory s = unicode"underflow ‬";
+    }
+}
+// ----
+// ParserError 8936: (88-106): Unicode direction override underflow in comment or string literal.

test/libsolidity/syntaxTests/literals/unicode_string_direction_override_2.sol

@@ -0,0 +1,9 @@
+contract C {
+    function f() public pure
+    {
+        // PDF PDF
+        bytes memory m = unicode"underflow ‬‬";
+    }
+}
+// ----
+// ParserError 8936: (92-110): Unicode direction override underflow in comment or string literal.

test/libsolidity/syntaxTests/literals/unicode_string_direction_override_3.sol

@@ -0,0 +1,9 @@
+contract C {
+    function f() public pure
+    {
+        // RLO
+        bytes memory m = unicode"overflow ‮";
+    }
+}
+// ----
+// ParserError 8936: (88-108): Mismatching directional override markers in comment or string literal.

test/libsolidity/syntaxTests/literals/unicode_string_direction_override_4.sol

@@ -0,0 +1,9 @@
+contract C {
+    function f() public pure
+    {
+        // RLO RLO
+        bytes memory m = unicode"overflow ‮‮";
+    }
+}
+// ----
+// ParserError 8936: (92-115): Mismatching directional override markers in comment or string literal.

test/libsolidity/syntaxTests/literals/unicode_string_direction_override_5.sol

@@ -0,0 +1,14 @@
+contract C {
+    function f() public pure
+    {
+        // RLO PDF
+        bytes memory m = unicode" ok ‮‬";
+
+        // RLO RLO PDF PDF
+        m = unicode" ok ‮‮‬‬";
+
+        // RLO RLO RLO PDF PDF PDF
+        m = unicode" ok ‮‮‮‬‬‬";
+    }
+}
+// ----

test/libsolidity/syntaxTests/literals/unicode_string_direction_override_6.sol

@@ -0,0 +1,9 @@
+contract C {
+    function f() public pure
+    {
+        // PDF RLO
+        bytes memory m = unicode" underflow ‬‮";
+    }
+}
+// ----
+// ParserError 8936: (92-111): Unicode direction override underflow in comment or string literal.

test/libsolidity/syntaxTests/literals/unicode_string_direction_override_7.sol

@@ -0,0 +1,13 @@
+contract C {
+    function f() public pure
+    {
+        // LRO PDF RLO PDF
+        bytes memory m = unicode"‭ ok ‬‮‬";
+
+        // lre rle pdf pdf
+        m = unicode"lre‪ rle‫ pdf‬ pdf‬";
+        // lre lro pdf pdf
+        m = unicode"lre‪ lro‭ pdf‬ pdf‬";
+    }
+}
+// ----