Use the uniform single line C++/99 style for code comments.

This is part of the cleanup proposed in "[RFC] compiler-rt builtins
cleanup and refactoring".

Differential Revision: https://reviews.llvm.org/D60352

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@359411 91177308-0d34-0410-b5e6-96231b3b80d8