Add user-supplied C runtime and compiler-rt library functions to
llvm.compiler.used to protect them from premature optimization by
passes like -globalopt and -ipsccp. Calls to (seemingly unused)
runtime library functions can be added by -instcombine and instruction
lowering.
Patch by Duncan Exon Smith, thanks!
Fixes <rdar://problem/14740087>
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194514 91177308-0d34-0410-b5e6-96231b3b80d8
