cpp -traditional -dM file.h misses most macros

Run clang's preprocessor with both `-traditional` and `-dM`, and it fails to dump most macros,

```
$ cat exam.h
#define MY1
/* line 2 */
#define MY3
```

Bad output from OpenBSD's packages of llvm-20.1.8p6 and llvm-21.1.8p4: MY3 is missing,

```
$ clang-cpp-20 -traditional -dM exam.h|grep MY
#define MY1
```

Good output from OpenBSD's packages of llvm-19.1.7p14 and gcc-15.2.0p5,

```
$ clang-cpp-19 -traditional -dM exam.h|grep MY
#define MY1
#define MY3
```

In general, -traditional -dM stops at any line that isn't a preprocessor directive, like `/* line 2 */` or `int i;` or an empty line. It never sees macros under that line.

I'm not sure, but I can guess the cause. -dM sets ShowCPP = false, ShowMacros = True, which causes clang/lib/Frontend/PrintPreprocessedOutput.cpp to call DoPrintMacros() and then LexTokensUntilEOF() in clang/lib/Lex/Preprocessor.cpp, which stops at a tok::unknown,

```cpp
void Preprocessor::LexTokensUntilEOF(std::vector *Tokens) {
while (1) {
Token Tok;
Lex(Tok);
if (Tok.isOneOf(tok::unknown, tok::eof, tok::eod,
tok::annot_repl_input_end))
break;
if (Tokens != nullptr)
Tokens->push_back(Tok);
}
}
```

but -traditional sets TraditionalCPP = true, which causes clang/lib/Lex/Lexer.cpp to SetKeepWhitespaceMode(true); and make tok::unknown from whitespace,

```cpp
// If the client wants us to return whitespace, return it now.
if (isKeepWhitespaceMode()) {
FormTokenWithChars(Result, CurPtr, tok::unknown);
```

So -traditional and -dM don't work together; the tok::unknown whitespace from -traditional acts like an end of file in -dM, which misses most macros.