[opt-viewer] Reduce memory consumption

The observation is that we have a lot of similar remarks with lots of
identical strings (e.g. file paths, text from the remark).  Storing a copy of
each of those strings in memory is wasteful.  This makes all the strings in
the remark interned which maintains a single immutable instance that is
referenced everywhere.

I get an average 20% heap size reduction with this but it's possible that this
varies with the typical length of the file paths used.  (I used heapy to
report the heap size.)  Runtime is same or a tiny bit better.

  | # of files            |   60 |  114 |  308 |  605 | 1370 |
  | # of remarks          |  20K |  37K | 146K | 180K | 640K |
  | total file size (MB)  |   22 |   51 |  219 |  202 | 1034 |
  |-----------------------+------+------+------+------+------|
  | Heap size before (MB) |  106 |  226 |  894 |  934 | 3573 |
  | Heap size after       |   86 |  179 |  694 |  739 | 2798 |
  | Rate                  | 0.81 | 0.79 | 0.78 | 0.79 | 0.78 |
  |-----------------------+------+------+------+------+------|
  | Average remark size   | 4.30 | 4.84 | 4.75 | 4.11 | 4.37 |
  | Mem2disk ratio        | 3.91 | 3.51 | 3.17 | 3.66 | 2.71 |

Differential Revision: https://reviews.llvm.org/D35534

llvm-svn: 308536