[clang] [clang] Fix include behavior for leading double slashes (PR #213689)

[Hubert Tong via cfe-commits]

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@@ -968,7 +968,7 @@ OptionalFileEntryRef HeaderSearch::LookupFile(
     *SuggestedModule = ModuleMap::KnownHeader();
 
   // If 'Filename' is absolute, check to see if it exists and no searching.
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hubert-reinterpretcast wrote:

Please expand the comment to explain what using `is_absolute_gnu` does here. In particular, please record the intention for whether Windows UNC paths are meant to be treated as "absolute" for the purposes of the comment.

https://github.com/llvm/llvm-project/pull/213689