[Clang]: prevent assertion on empty filename arg in __has_embed (#159928)

Fixes #159898

---

This PR addresses the issue of Clang asserting when `__has_embed` is
used with an empty filename

```c

#if __has_embed("")
#endif
```

Author: a-tarasyuk

clang/docs/ReleaseNotes.rst

@@ -366,6 +366,7 @@ Bug Fixes in This Version
 - Fixed an assertion when an improper use of the ``malloc`` attribute targeting
   a function without arguments caused us to try to access a non-existent argument.
   (#GH159080)
+- Fixed a failed assertion with empty filename arguments in ``__has_embed``. (#GH159898)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Lex/PPMacroExpansion.cpp

@@ -1282,11 +1282,13 @@ EmbedResult Preprocessor::EvaluateHasEmbed(Token &Tok, IdentifierInfo *II) {
 
   SmallString<128> FilenameBuffer;
   StringRef Filename = this->getSpelling(FilenameTok, FilenameBuffer);
+  if (Filename.empty())
+    return EmbedResult::Empty;
+
   bool isAngled =
       this->GetIncludeFilenameSpelling(FilenameTok.getLocation(), Filename);
   // If GetIncludeFilenameSpelling set the start ptr to null, there was an
   // error.
-  assert(!Filename.empty());
   const FileEntry *LookupFromFile =
       this->getCurrentFileLexer() ? *this->getCurrentFileLexer()->getFileEntry()
                                   : static_cast<FileEntry *>(nullptr);

clang/test/Preprocessor/embed___has_embed_parsing_errors.c

@@ -247,3 +247,6 @@
    expected-error@+2 {{expected value in expression}}
 #if __has_embed (__FILE__ limit(-100000000000000000)) != __STDC_EMBED_NOT_FOUND__
 #endif
+
+#if __has_embed("") // expected-error {{empty filename}}
+#endif