Simplify pragma keep handling for forward declarations

Rename ForwardDeclareInKeepRange to ForwardDeclareIsMarkedKeep, take a
NamedDecl and move all location and text wrangling into the function.

This is a layering violation in a sense, because the preprocessor
doesn't know anything about declarations. But the API is much cleaner
that way from the callers' perspective -- they typically just have a
decl and shouldn't be trusted to use the right location info.

No functional change.

iwyu.cc

@@ -3987,13 +3987,8 @@ class IwyuAstConsumer
               definitely_keep_fwd_decl = true;
           }
         }
-      } else {
-        SourceLocation decl_end_location = decl->getSourceRange().getEnd();
-        if (LineHasText(decl_end_location, "// IWYU pragma: keep") ||
-            LineHasText(decl_end_location, "/* IWYU pragma: keep") ||
-            preprocessor_info().ForwardDeclareInKeepRange(decl_end_location)) {
-          definitely_keep_fwd_decl = true;
-        }
+      } else if (preprocessor_info().ForwardDeclareIsMarkedKeep(decl)) {
+        definitely_keep_fwd_decl = true;
       }
 
       preprocessor_info().FileInfoFor(CurrentFileEntry())->AddForwardDeclare(

iwyu_preprocessor.cc

@@ -28,6 +28,7 @@
 #include "iwyu_string_util.h"
 #include "iwyu_verrs.h"
 #include "llvm/Support/raw_ostream.h"
+#include "clang/AST/Decl.h"
 #include "clang/Basic/IdentifierTable.h"
 #include "clang/Lex/MacroInfo.h"
 
@@ -37,6 +38,7 @@ using clang::FileID;
 using clang::MacroDefinition;
 using clang::MacroDirective;
 using clang::MacroInfo;
+using clang::NamedDecl;
 using clang::Preprocessor;
 using clang::SourceLocation;
 using clang::SourceRange;
@@ -1123,7 +1125,12 @@ bool IwyuPreprocessorInfo::ForwardDeclareIsInhibited(
       ContainsKey(*inhibited_forward_declares, normalized_symbol_name);
 }
 
-bool IwyuPreprocessorInfo::ForwardDeclareInKeepRange(SourceLocation loc) const {
+bool IwyuPreprocessorInfo::ForwardDeclareIsMarkedKeep(
+    const NamedDecl* decl) const {
+  // Use end-location so that any trailing comments match only on the last line.
+  SourceLocation loc = decl->getEndLoc();
+
+  // Is the decl part of a begin_keep/end_keep block?
   const FileEntry* file = GetFileEntry(loc);
   auto keep_ranges = keep_location_ranges_.equal_range(file);
   for (auto it = keep_ranges.first; it != keep_ranges.second; ++it) {
@@ -1131,7 +1138,8 @@ bool IwyuPreprocessorInfo::ForwardDeclareInKeepRange(SourceLocation loc) const {
       return true;
     }
   }
-  return false;
+  // Is the declaration itself marked with trailing comment?
+  return (LineHasText(loc, "// IWYU pragma: keep") ||
+          LineHasText(loc, "/* IWYU pragma: keep"));
 }
-
 }  // namespace include_what_you_use

iwyu_preprocessor.h

@@ -79,6 +79,7 @@
 namespace clang {
 class FileEntry;
 class MacroInfo;
+class NamedDecl;
 }  // namespace clang
 
 namespace include_what_you_use {
@@ -168,9 +169,8 @@ class IwyuPreprocessorInfo : public clang::PPCallbacks,
   bool ForwardDeclareIsInhibited(
       const clang::FileEntry* file, const string& qualified_symbol_name) const;
 
-  // Return true if the fwd decl is in the range of a begin_keep -> end_keep
-  // block.
-  bool ForwardDeclareInKeepRange(clang::SourceLocation loc) const;
+  // Return true if the fwd decl is marked with "IWYU pragma: keep".
+  bool ForwardDeclareIsMarkedKeep(const clang::NamedDecl* decl) const;
 
  protected:
   // Preprocessor event handlers called by Clang.