[clang-format] Handle "complex" conditionals in RemoveBracesLLVM

Do not remove braces if the conditional of if/for/while might not
fit on a single line even after the opening brace is removed.

Examples:
// ColumnLimit: 20
// 45678901234567890
if (a) { /* Remove. */
  foo();
}
if (-b >= c) { // Keep.
  bar();
}

Differential Revision: https://reviews.llvm.org/D126052