folly/cli: eliminate double scan in cli_parse_args_from_content EOF path

[darion-yaphet]

The code that handles a token still in current when content ends had two independent backward scans:

1. A while/pop_back loop that stripped trailing spaces from the processed string value (current).
2. A for loop that scanned content backward from EOF to recompute the byte-span length, because current may have undergone escape substitution and its size no longer matches the raw source bytes.

The root cause was that the main scan loop had no record of where in content the last non-whitespace character of the current token sat.

Fix: add token_end_offset (byte index into content of the last non-space character), updated in every branch that already updates token_end_line/token_end_col — seven sites in total. The EOF path can now compute length = token_end_offset + 1 - token_start in O(1), removing the backward scan of content entirely. The while/pop_back trim of current is kept (it handles the string value) but tightened to the idiomatic while (!x.empty() && cond(x.back())) form.