Switch from C-style comments `/* ... */` to C++ style `//` (NFC)

This is mostly a scripted update, it may not be perfect.

function replace() {
  FROM=$1
  TO=$2
  git grep "$FROM" $REPO_PATH |cut -f 1 -d : | sort -u | \
    while read file; do
      sed -i "s#$FROM#$TO#" $file ;
    done
}

replace '|\*===----------------------------------------------------------------------===\*|$' '//===----------------------------------------------------------------------===//'
replace '^/\* =' '//=='
replace '^/\*=' '//='
replace '^\\\*=' '//='
replace '^|\*' '//'
replace ' \*|$' ''
replace '=\*\\$' '=//'
replace '== \*/$' '===//'
replace '==\*/$' '==//'
replace '^/\*\*\(.*\)\*/$' '///\1'
replace '^/\*\(.*\)\*/$' '//\1'
replace '//============================================================================//' '//===----------------------------------------------------------------------===//'

Differential Revision: https://reviews.llvm.org/D90732