[clang] [clang-format] Handle Trailing Whitespace After Line Continuation (P2223R2) (PR #145243)

[Owen Pan via cfe-commits]

================
@@ -1205,14 +1206,23 @@ static size_t countLeadingWhitespace(StringRef Text) {
   while (Cur < End) {
     if (isspace(Cur[0])) {
       ++Cur;
-    } else if (Cur[0] == '\\' && (Cur[1] == '\n' || Cur[1] == '\r')) {
-      // A '\' followed by a newline always escapes the newline, regardless
-      // of whether there is another '\' before it.
+    } else if (Cur[0] == '\\') {
+      // A '\' followed by a optional horizontal whitespace (P22232R2) and then
+      // newline  always escapes the newline, regardless  of whether there is
+      // another '\' before it.
       // The source has a null byte at the end. So the end of the entire input
       // isn't reached yet. Also the lexer doesn't break apart an escaped
       // newline.
-      assert(End - Cur >= 2);
-      Cur += 2;
+      const unsigned char *Lookahead = Cur + 1;
+      while (isHorizontalWhitespace(*Lookahead))
+        ++Lookahead;
+      if (*Lookahead == '\n' || *Lookahead == '\r') {
+        // Splice found, consume it.
+        Cur = Lookahead + 1;
+        continue;
+      }
+      // No line splice found; the '\' is a token.
+      break;
----------------
owenca wrote:

```suggestion
      // No line splice found; the backslash is a token.
      if (!isVerticalWhitespace(*Lookahead))
        break;
      // Splice found, consume it.
      Cur = Lookahead + 1;
```

https://github.com/llvm/llvm-project/pull/145243