[Issue 19266] Some source files names are no longer accepted

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19266

--- Comment #3 from github-bugzilla at puremagic.com ---
Commit pushed to master at https://github.com/dlang/dmd

https://github.com/dlang/dmd/commit/ac9272cd42a3a2c10639217abf98b166f9b12baf
fix issue 19266 - Some source files names are no longer accepted

only add the \\?\ prefix if the filename is actually long and does not start
with \\ already.

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