fix panic when rustc tries to reduce intermediate filenames len with utf8
The issue cannot be reproduced with the former testcase of creating external crates because rust refuses to use "external crate 28_找出字符串中第一个匹配项的下标" because it is not a valid indentifier (starts with number, and contain non ascii chars)
But still using 28_找出字符串中第一个匹配项的下标.rs as a filename is accepted by previous rustc releases So we consider it valid, and add an integration test for it to catch any regression on other code related to non ascii filenames.
Fixrust-lang/rust#147975
2aaa62b89d