fixed ISPC compilation fail caused by LLVM DIBuilder API change

fixed ISPC compilation fail caused by the change of llvm::Value to llvm::Constant in DIBuilder member functions (LLVM commit 222070, https://llvm.org/svn/llvm-project/llvm/trunk@222070)

TODO:
The testing has revealed 7 new runfails and 126 new compfails (comparing to only 40 compfails before the build failure). Need to examine and fix those compfails (the 93 new ones are on generic-4 and generic-16 and 40 old ones are on sse2-i32x4)

Details:
x86          sse2-i32x4   Linux LLVM 3.6 clang++3.6 -O2. Fails: 20, New fails: 20.
x86-64     sse2-i32x4   Linux LLVM 3.6 clang++3.6 -O2. Fails: 20, New fails: 20.
x86-64     generic-4     Linux LLVM 3.6 clang++3.6 -O2. Fails: 16, New fails: 16. 
x86-64     generic-4     Linux LLVM 3.6 clang++3.6 -O0. Fails: 34, New fails: 34.
x86-64     generic-16   Linux LLVM 3.6 clang++3.6 -O2. Fails: 10, New fails: 10.
x86-64     generic-16   Linux LLVM 3.6 clang++3.6 -O0. Fails: 33, New fails: 33.