[Doxygen] Ignore llvm::Expected<T> forward decl so doxgen recommends right header file

[abrachet]

Summary

The documentation for llvm::Expected currently suggests that the header file for Expected is Support/CachePruning.h, this is wrong, adding \cond and \endcond forces doxygen to ignore this forward declaration. This does work, and the outputted documentation correctly recommends Support/Error.h, but it is also an ugly addition we shouldn't have to make. This is kind of an unwinnable battle.

[jhenderson]

I don't really know anything about Doxygen, so I don't know if I can really review this issue. However, it seems to me like this isn't a good method, because it's likely only one "bad" place, and we'd have to litter everywhere with these @cond expressions, when in fact it's Doxygen picking a declaration rather than a definition of a class that's the problem.

[abrachet]

I agree. I looked if doxygen had any flags to ignore forward declarations, it seems like there isn’t. It’s certainly true that if it can’t be done on doxygens end it would be impossible for us to track all of these down, and as you mention super ugly for the code base. I’ve even seen it get tripped up on friend declarations!

I’m hopeful clang-doc won’t have these issues :)

llvm/include/llvm/Support/CachePruning.h

 #ifndef LLVM_SUPPORT_CACHE_PRUNING_H
 #define LLVM_SUPPORT_CACHE_PRUNING_H
 
 #include "llvm/ADT/StringRef.h"
 #include <chrono>
 
 namespace llvm {
 
+// Ignore forward declaration to not confuse Doxygen output.
+/// @cond
 template <typename T> class Expected;
+/// @endcond
 
 /// Policy for the pruneCache() function. A default constructed
 /// CachePruningPolicy provides a reasonable default policy.
 struct CachePruningPolicy {
   /// The pruning interval. This is intended to be used to avoid scanning the
   /// directory too often. It does not impact the decision of which file to
   /// prune. A value of 0 forces the scan to occur. A value of None disables
   /// pruning.