[clang][HeaderSearch] Make sure there are no backslashes in suggestedPath

[sammccall]

This looks like a bug rather than a feature of HeaderSearch, shouldn't the slash conversion go there?

(It seems unlikely that diagnostics should suggest one thing but we insert another)

[kadircet]

I had just looked at the method name and thought it was the right place( suggestPathToFileForDiagnostics since it says "path to file".)

But looking at the comments,

/// Suggest a path by which the specified file could be found, for
/// use in diagnostics to suggest a #include.

HeaderSearch API itself seems like a better place, I suppose the confusion comes from the fact that paths and the include-directives are
the same on unix-like systems.

Updating the patch

[kadircet]

• Fix bug in HeaderSearch instead of clangd

[sammccall]

Thanks!
Can you document this in HeaderSearch.h?

[kadircet]

• Update documentation

[sammccall]

clang/include/clang/Lex/HeaderSearch.h

710 ↗

(On Diff #196392)

I don't think this is specific enough - on windows, backslashes are valid (or at least accepted by clang).
I'd suggest specifically mentioning forward slashes.

[kadircet]

• Update comments to focus on forward slashes

clang-tools-extra/clangd/Headers.cpp

 std::string
 IncludeInserter::calculateIncludePath(const HeaderFile &InsertedHeader) const {
   assert(InsertedHeader.valid());
   if (InsertedHeader.Verbatim)
     return InsertedHeader.File;
   bool IsSystem = false;
   if (!HeaderSearchInfo)
     return "\"" + InsertedHeader.File + "\"";
-  std::string Suggested = HeaderSearchInfo->suggestPathToFileForDiagnostics(
-      InsertedHeader.File, BuildDir, &IsSystem);
+  // Suggest path might contain back slashes on windows.
+  std::string Suggested = llvm::sys::path::convert_to_slash(
+      HeaderSearchInfo->suggestPathToFileForDiagnostics(InsertedHeader.File,
+                                                        BuildDir, &IsSystem));
   if (IsSystem)
     Suggested = "<" + Suggested + ">";
   else
     Suggested = "\"" + Suggested + "\"";
   return Suggested;
 }
 
 llvm::Optional<TextEdit>