Build fix for the latest LLVMContext changes (LLVM r75445) This shouldn't break the build with older LLVM revs. We include LLVMContext.h in gen/llvm.h now to make the transition a little bit easier.

module bug1;
struct Foo { Foo opSub(ref Foo b) { return Foo(); } }
struct Bar { Foo whee; }
void test(ref Bar moo) { Foo nngh; auto plonk = nngh - moo.whee; }
void main() { Bar bar; test(bar); }