[clangd] Fix opening declarations located in non-preamble inclusion

[malaperle]

Summary

When an inclusion is not processed as part of the preamble, its path is
not made into an absolute path as part of the precompiled header code
(adjustFilenameForRelocatableAST in ASTWriter.cpp). Because of this,
when we convert a Decl location to retrieve the file name with
FileEntry->getName(), it is possible for this path to be relative.
Instead, we should try to use tryGetRealPathName first which returns
an absolute path.

Fixes bug 35217.

[sammccall]

LGTM

clangd/ClangdUnit.cpp

970 ↗

(On Diff #121803)

Can you add a comment about when this is needed?
like // Non-preamble included files may have relative paths.

clang-tools-extra/trunk/clangd/ClangdUnit.cpp

void addDeclarationLocation(const SourceRange &ValSourceRange) {
     Position Begin;
     Begin.line = SourceMgr.getSpellingLineNumber(LocStart) - 1;
     Begin.character = SourceMgr.getSpellingColumnNumber(LocStart) - 1;
     Position End;
     End.line = SourceMgr.getSpellingLineNumber(LocEnd) - 1;
     End.character = SourceMgr.getSpellingColumnNumber(LocEnd) - 1;
     Range R = {Begin, End};
     Location L;
-    L.uri = URI::fromFile(
-        SourceMgr.getFilename(SourceMgr.getSpellingLoc(LocStart)));
+    if (const FileEntry *F =
+            SourceMgr.getFileEntryForID(SourceMgr.getFileID(LocStart))) {
+      StringRef FilePath = F->tryGetRealPathName();
+      if (FilePath.empty())
+        FilePath = F->getName();
+      L.uri = URI::fromFile(FilePath);
       L.range = R;
       DeclarationLocations.push_back(L);
     }
+  }
 
   void finish() override {
     // Also handle possible macro at the searched location.
     Token Result;
     if (!Lexer::getRawToken(SearchedLocation, Result, AST.getSourceManager(),
                             AST.getLangOpts(), false)) {
       if (Result.is(tok::raw_identifier)) {
         PP.LookUpIdentifierInfo(Result);