Make the visibility of LLVMPPCCompilationCallback work with GCC.

GCC warns about the attribute being ignored if it occurs after void*.
There seems to be some kind of incompatibility between clang and gcc here, but
I can't fathom who's right.

void* LLVM_LIBRARY_VISIBILITY foo(); // clang: hidden, gcc: default
LLVM_LIBRARY_VISIBILITY void *bar(); // clang: hidden, gcc: hidden
void LLVM_LIBRARY_VISIBILITY qux();  // clang: hidden, gcc: hidden