Adjust the comment section of CreateSource to account for lines longer than 60

Summary:
On rare occasions, the address, instruction and arguments of a line of
assembly in the CreateSource printout would reach > 60 characters. The
function would integer overflow and try to indent a line by `0xfff...`.

Change the calculated offset to be the maximum of 60 or
`line_strm.str().size()`

Reviewers: clayborg, xiaobai

Reviewed By: clayborg

Differential Revision: https://reviews.llvm.org/D52745

llvm-svn: 346179