Use StringBuilder instead of concatenation in SubmoduleOp

Coverity reports a performance issue caused by using the + operator
to concatenate strings within a loop. (See CID 20152) on the coverity
scan project [1].

Refactor it to use a StringBuilder instead.

[1] https://scan.coverity.com/projects/4032

Change-Id: Ib2ee90934e2215639cadc500ae28c9e61f8a9d19
1 file changed