[SimplifyLibcalls] Replace locked IO with unlocked IO

Summary: If file stream arg is not captured and source is fopen, we could replace IO calls by unlocked IO ("_unlocked" function variants) to gain better speed,

Reviewers: efriedma, RKSimon, spatel, sanjoy, hfinkel, majnemer

Subscribers: lebedev.ri, llvm-commits

Differential Revision: https://reviews.llvm.org/D45736

llvm-svn: 331002