[include-cleaner] Weaken signal for boosting preferred headers

[kadircet]

Summary

Putting preferred header signal above completeness implied we would
uprank forward declarations above complete ones in certain cases.

This can be desired in cases where:

• Complete definition is private. But this case is already governed by publicness signal.
• The library indeed intends to provide a forward declaring interface, like iosfwd vs ostream.

In all other cases, upranking is undesired as it means we've picked up prefered
headerness signal by mistake from an unrelated declaration to the library.

This change regresses the behavior for libraries that intentionally provide a
forward declaring interface. But that wasn't something we intended to support
explicitly, it was working incidentally when the forward declaring header had a
similar name to the symbol. Moreover, include-cleaner deliberately discourages
forward-declarations, so not working in this case is also more aligned with rest
of the components.

[kadircet]

see also https://github.com/llvm/llvm-project/issues/62172

clang-tools-extra/include-cleaner/lib/TypesInternal.h

 /// any public definition, even if it also provides private ones.
 ///
 /// Hints are sorted in ascending order of relevance.
 enum class Hints : uint8_t {
   None = 0x00,
   /// Symbol is directly originating from this header, rather than being
   /// exported or included transitively.
   OriginHeader = 1 << 0,
-  /// Provides a generally-usable definition for the symbol. (a function decl,
-  /// or class definition and not a forward declaration of a template).
-  CompleteSymbol = 1 << 1,
   /// Header providing the symbol is explicitly marked as preferred, with an
   /// IWYU private pragma that points at this provider or header and symbol has
   /// ~the same name.
-  PreferredHeader = 1 << 2,
+  PreferredHeader = 1 << 1,
+  /// Provides a generally-usable definition for the symbol. (a function decl,
+  /// or class definition and not a forward declaration of a template).
+  CompleteSymbol = 1 << 2,
   /// Symbol is provided by a public file. Only absent in the cases where file
   /// is explicitly marked as such, non self-contained or IWYU private
   /// pragmas.
   PublicHeader = 1 << 3,
   LLVM_MARK_AS_BITMASK_ENUM(PublicHeader),
 };
 LLVM_ENABLE_BITMASK_ENUMS_IN_NAMESPACE();
 /// A wrapper to augment values with hints.

clang-tools-extra/include-cleaner/unittests/FindHeadersTest.cpp

Inputs.ExtraFiles["bar.h"] = guard(R"cpp(
     void foo();
   )cpp");
   buildAST();
   EXPECT_THAT(headersForFoo(),
               ElementsAre(physicalHeader("bar.h"), physicalHeader("fox.h")));
 }
 
 TEST_F(HeadersForSymbolTest, Ranking) {
-  // Sorting is done over (canonical, public, complete, origin)-tuple.
+  // Sorting is done over (public, complete, canonical, origin)-tuple.
   Inputs.Code = R"cpp(
     #include "private.h"
     #include "public.h"
     #include "public_complete.h"
     #include "exporter.h"
   )cpp";
   Inputs.ExtraFiles["public.h"] = guard(R"cpp(
     struct foo;
   )cpp");
   Inputs.ExtraFiles["private.h"] = guard(R"cpp(
     // IWYU pragma: private, include "canonical.h"
     struct foo;
   )cpp");
   Inputs.ExtraFiles["exporter.h"] = guard(R"cpp(
   #include "private.h" // IWYU pragma: export
   )cpp");
   Inputs.ExtraFiles["public_complete.h"] = guard("struct foo {};");
   buildAST();
-  EXPECT_THAT(headersForFoo(), ElementsAre(Header("\"canonical.h\""),
-                                           physicalHeader("public_complete.h"),
-                                           physicalHeader("public.h"),
-                                           physicalHeader("exporter.h"),
-                                           physicalHeader("private.h")));
+  EXPECT_THAT(headersForFoo(),
+              ElementsAre(physicalHeader("public_complete.h"),
+                          Header("\"canonical.h\""), physicalHeader("public.h"),
 }
 
 TEST_F(HeadersForSymbolTest, PreferPublicOverComplete) {
   Inputs.Code = R"cpp(
     #include "complete_private.h"
     #include "public.h"
   )cpp";
   Inputs.ExtraFiles["complete_private.h"] = guard(R"cpp(
     // IWYU pragma: private
     struct foo {};
   )cpp");
   Inputs.ExtraFiles["public.h"] = guard("struct foo;");
   buildAST();
   EXPECT_THAT(headersForFoo(),
               ElementsAre(physicalHeader("public.h"),
                           physicalHeader("complete_private.h")));
 }
 
 TEST_F(HeadersForSymbolTest, PreferNameMatch) {
   Inputs.Code = R"cpp(
     #include "public_complete.h"
     #include "test/foo.fwd.h"
   )cpp";
-  Inputs.ExtraFiles["public_complete.h"] = guard(R"cpp(
-    struct foo {};
-  )cpp");
+  Inputs.ExtraFiles["public_complete.h"] = guard("struct foo {};");
   Inputs.ExtraFiles["test/foo.fwd.h"] = guard("struct foo;");
   buildAST();
-  EXPECT_THAT(headersForFoo(),
-              ElementsAre(physicalHeader("test/foo.fwd.h"),
-                          physicalHeader("public_complete.h")));
+  EXPECT_THAT(headersForFoo(), ElementsAre(physicalHeader("public_complete.h"),
+                                           physicalHeader("test/foo.fwd.h")));
 }
 
 TEST_F(HeadersForSymbolTest, MainFile) {
   Inputs.Code = R"cpp(
     #include "public_complete.h"
     struct foo;
   )cpp";
   Inputs.ExtraFiles["public_complete.h"] = guard(R"cpp(