[tclquadcode] LLVM 7 support

[Donal K. F. <don...@ma...>]

I've just published an update to llvmtcl to support LLVM 7; llvmtcl 
3.9.2 is a very small change, but LLVM 7 is a slightly larger change 
that's forced me to do a small update to the code generator. The issue 
is that the type signatures of two intrinsics (for memcpy and memset) 
have changed; the new code is flexible between LLVM versions.

People using branches other than trunk will want to merge trunk before 
using LLVM 7. (llvmtcl 3.9.2 also removes one API call, but we weren't 
making any use of it.)

Donal.