Paths that start with `\\?\` are absolute paths, and aren't expected
to be used with wildcard expressions.
Previously, the `?` at the start of the path triggered the condition
for a potential wildcard, which caused the path to be split and
reassembled. In builds with `LLVM_WINDOWS_PREFER_FORWARD_SLASH=ON`,
this caused a path like e.g. `\\?\D:\tmp\hello.cpp` to be reassembled
into `\\?\D:\tmp/hello.cpp` which isn't a valid path (as such
absolute paths must use backslashes consistently).
This fixes https://github.com/mstorsjo/llvm-mingw/issues/280.
I'm not sure if there's any straightforward way to add a test
for this case, unfortunately.
Differential Revision: https://reviews.llvm.org/D126675
7e2afe83e8