[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])) {
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owenca wrote:

```suggestion
    if (isWhitespace(Cur[0])) {
```

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