Dwarf: use utostr on CUID to append to SmallString.

We used to do "SmallString += CUID", which is incorrect, since CUID will
be truncated to a char.

rdar://problem/13573833


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178941 91177308-0d34-0410-b5e6-96231b3b80d8