Scanner: Many int64 fixes, including performance.

- partially addresses #1209 and #1187
- only query 64-bit extension on seeing use of a 64-bit literal
  (was doing it for every single token)
- correct HLSL acceptance of 64-bit literal syntax (still an int though)
- error on overflow of 32-bit literal type

Test/spv.atomicInt64.comp

@@ -22,7 +22,7 @@ shared Struct s;
 void main()
 {
     const int64_t  i64c = -24;
-    const uint64_t u64c = 0xF00000000F; 
+    const uint64_t u64c = 0xF00000000Ful; 
 
     // Test shader storage block
     int64_t  i64 = 0;