Rename String.concat() implementation function.

Rename String::AllocFromStrings() to String::DoConcat() and
rename arguments to h_this and h_arg.

This addresses a comment from
    https://android-review.googlesource.com/1473728 .

Test: testrunner.py --host --optimizing -t 021-string2
Bug: 169674485
Change-Id: I830541b64b51c0170aa5455cbad7cc86f4349f13
3 files changed