Allow hex string literals in Yul.

test/libsolidity/syntaxTests/inlineAssembly/hex_assignment.sol

@@ -6,4 +6,3 @@ contract C {
   }
 }
 // ----
-// ParserError 3772: (72-81): Hex literals are not valid in this context.

test/libsolidity/syntaxTests/inlineAssembly/hex_expression.sol

@@ -6,4 +6,3 @@ contract C {
   }
 }
 // ----
-// ParserError 3772: (67-76): Hex literals are not valid in this context.

test/libsolidity/syntaxTests/inlineAssembly/hex_switch_case.sol

@@ -8,4 +8,3 @@ contract C {
   }
 }
 // ----
-// ParserError 3772: (92-99): Hex literals are not valid in this context.

test/libsolidity/syntaxTests/literals/hex_string_duplicate_underscore_yul.sol

@@ -0,0 +1,9 @@
+contract C {
+    function f() public pure {
+        assembly {
+            let x := hex"12__34";
+        }
+    }
+}
+// ----
+// ParserError 1465: (84-92): Illegal token: Invalid use of number separator '_'.

test/libsolidity/syntaxTests/literals/hex_string_invalid_characters_yul.sol

@@ -0,0 +1,9 @@
+contract C {
+    function f() public pure {
+        assembly {
+            let x := hex"abxy";
+        }
+    }
+}
+// ----
+// ParserError 1465: (84-90): Illegal token: Expected even number of hex-nibbles.

test/libsolidity/syntaxTests/string/hex_as_identifier.sol

@@ -0,0 +1,5 @@
+function g() pure {
+    assembly { let hex := 1 }
+}
+// ----
+// ParserError 2314: (39-42): Expected identifier but got 'ILLEGAL'