RFR: 8293251: Use stringStream::base() instead of as_string() when applicable [v5]

[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 two additional commits since the last revision:

 - Change base to internal_string
 - Add invariant tracking in new method

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10142/files
  - new: https://git.openjdk.org/jdk/pull/10142/files/700f91b4..4685ff32

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10142&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10142&range=03-04

  Stats: 62 lines in 27 files changed: 8 ins; 0 del; 54 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