spanner/spansql: support IF NOT EXISTS in DDL parsing

**Client**
spanner/spansql

**Environment**

N/A

**Go Environment**

N/A

**Code**

N/A

**Expected behavior**
`spansql.ParseDDL` should support statements like `CREATE TABLE IF NOT EXISTS` since that is now supported in Spanner.

**Actual behavior**

Error is being thrown:
```
got "NOT" while expecting "("
```
