sql: Adding support for integral aliases.

Before this patch we would discard more precise type informations for types
such as INT8 and INT64. SHOW CREATE TABLE would show INT even if the user
specified either of those.

After this patch we'll save this information. SHOW CREATE TABLE will now
display the correct type.

pkg/sql/logictest/testdata/logic_test/table

@@ -352,8 +352,8 @@ SHOW COLUMNS FROM test.alltypes
 Field  Type                      Null   Default         Indices
 a      BOOL                      true   NULL            {}
 b      INT                       true   NULL			{}
-c      INT                       true   NULL			{}
-d      INT                       true   NULL			{}
+c      INT8                      true   NULL			{}
+d      INT64                     true   NULL			{}
 e      INT                       true   NULL			{}
 f      INT                       true   NULL			{}
 g      INT                       true   NULL			{}

pkg/sql/sqlbase/structured.go

@@ -1742,6 +1742,9 @@ func (c *ColumnType) SQLString() string {
 	case ColumnType_INT_ARRAY:
 		return "INT[]"
 	}
+	if c.Alias != ColumnType_NONE {
+		return c.Alias.String()
+	}
 	return c.Kind.String()
 }