RFR: 8293251: Use stringStream::base() instead of as_string() when applicable [v4]

[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:

  Remove invariant tracking
  
  Too much depends on this invariant not being met.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10142/files
  - new: https://git.openjdk.org/jdk/pull/10142/files/0532bedf..700f91b4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10142&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10142&range=02-03

  Stats: 3 lines in 2 files changed: 0 ins; 3 del; 0 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