This check flags uses of `std::endl` on streams and suggests using the newline character `'\n'` instead. `std::endl` performs two operations: it writes a newline character to the output stream and then flushes the stream buffer, which can be less efficient than writing a single newline character using `'\n'`.
This fixes llvm#35321
Reviewed By: PiotrZSL
Differential Revision: https://reviews.llvm.org/D148318
