[clang-tools-extra] Add clangd-index-server support for cross platform indexes (PR #207202)

[Aleksandr Platonov via cfe-commits]

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@@ -0,0 +1,9 @@
+# Verify that clangd-index-server accepts Windows-style absolute paths as
+# project root, allowing the server to run on Linux with a Windows-built index.
+# "C:/project" is absolute in Windows style but not POSIX style.
+# REQUIRES: clangd-remote-index
+# RUN: rm -f %t.idx
+# RUN: not clangd-index-server --server-address=localhost:0 %t.idx "C:/project" 2>&1 | FileCheck %s
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ArcsinX wrote:

Can we use backslash in Windows path?

It's better to test Windows paths in their native form.

https://github.com/llvm/llvm-project/pull/207202