Throw FATAL_ERROR for addrspacecast in a constant expression
Throw FATAL_ERROR for addrspacecast in a constant expression

These may be generated due to a bug in LLVM 4.0 (fixed in 5.0). Better
to die with a (semi-)useful error message rather than segfaulting.

mpflanzer/Oclgrind,mpflanzer/Oclgrind,mpflanzer/Oclgrind,mpflanzer/Oclgrind