[clang] [clang-scan-deps] Fix "unterminated conditional directive" bug (PR #146645)

[Naveen Seth Hanig via cfe-commits]

================
@@ -393,7 +393,7 @@ static bool isQuoteCppDigitSeparator(const char *const Start,
 }
 
 void Scanner::skipLine(const char *&First, const char *const End) {
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naveen-seth wrote:

My bad! The bug was introduced because I initialized `char LastNonWhitespace = ' ';` one scope too far out.
This can cause `LastNonWhitespace` to not be reset (it remains `'\'`), which then results in an additional line being skipped.

To fix this bug, the only change needed is to move it one scope down, into the for loop:
```cpp
void Scanner::skipLine(const char *&First, const char *const End) {
  for (;;) {
    assert(First <= End);
    if (First == End)
      return;

    if (isVerticalWhitespace(*First)) {
      skipNewline(First, End);
      return;
    }
    const char *Start = First;
    char LastNonWhitespace = ' ';
    while (First != End && !isVerticalWhitespace(*First)) {
```

https://github.com/llvm/llvm-project/pull/146645