[clang] [Clang][Preprocessor] Unify header-name lookahead for import and include (PR #191004)

[Richard Smith via cfe-commits]

================
@@ -4695,11 +4696,17 @@ bool Lexer::LexDependencyDirectiveToken(Token &Result) {
     MIOpt.ReadToken();
   }
 
-  if (ParsingFilename && DDTok.is(tok::less)) {
-    BufferPtr = BufferStart + DDTok.Offset;
-    LexAngledStringLiteral(Result, BufferPtr + 1);
-    if (Result.isNot(tok::header_name))
+  const char *DDTokPtr = BufferStart + DDTok.Offset;
+  if (ParsingFilename && *DDTokPtr == '<') {
----------------
zygoloid wrote:

What happens if the `<` is immediately after an escaped newline? Eg:
```
... \
<foo>
```
I *think* we use the location of the `\` as the `DDTok.Offset` in this case. Worth adding a testcase. If it fails, this would be the fix:

```suggestion
  const char *DDTokPtr = SkipEscapedNewLines(BufferStart + DDTok.Offset);
  if (ParsingFilename && *DDTokPtr == '<') {
```

https://github.com/llvm/llvm-project/pull/191004