RFR: 8293251: Use stringStream::base() instead of as_string() when applicable [v2]

[Johan Sjölén]

> Hi!
> 
> Please review this PR swapping out stringStream::as_string() with ::base() when applicable. With this change we avoid allocating a resource managed string copy. I also attempt to document the base function, as it is not safe to use any result returned from it if you subsequently call the stringStream's methods.
> 
> When I've left ResourceMarks in place I've also commented which calls requires them.
> 
> This passes tier1, tier2 tests.

Johan Sjölén has updated the pull request incrementally with one additional commit since the last revision:

  Fix review comments

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/10142/files
  - new: https://git.openjdk.org/jdk/pull/10142/files/2fe12418..24dd048c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10142&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10142&range=00-01

  Stats: 8 lines in 3 files changed: 1 ins; 2 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/10142.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10142/head:pull/10142

PR: https://git.openjdk.org/jdk/pull/10142