Fixes large rational number literals being wrongly interpreted.

Fixes #5052.

test/libsolidity/syntaxTests/types/rational_number_huge.sol

@@ -0,0 +1,10 @@
+contract C {
+    function f(uint y) public pure {
+        // fits FixedBytes with exactly 32-bytes
+        y = 0xffffffff00000000ffffffff00000000ffffffff00000000ffffffff00000000; // FixedBytes (32)
+
+        // fits exactly into FixedBytes (32), ensures underscored literals won't hurt
+        y = 0xffffffff00000000ffffffff00000000ffffffff00000000ffffffff_00000000;
+    }
+}
+// ----

test/libsolidity/syntaxTests/types/rational_number_huge_fail.sol

@@ -0,0 +1,8 @@
+contract C {
+    function f(uint y) public pure {
+        // one byte too long for storing in Fixedbytes (would require 33 bytes)
+        y = 0xffffffff00000000ffffffff00000000ffffffff00000000ffffffff000000001;
+    }
+}
+// ----
+// TypeError: (142-209): Type int_const 1852...(71 digits omitted)...7281 is not implicitly convertible to expected type uint256.