compile all under LLVM 9

[ctaggart]

"all compilers and the linker involved must be compatible versions" to get cross-language LTO working. I wonder if something similar is needed to really get this working.

http://blog.llvm.org/2019/09/closing-gap-cross-language-lto-between.html

rust master currently targets LLVM 9.0 2019-09-19
https://github.com/rust-lang/rust/blob/master/.gitmodules#L40-L43

[submodule "src/llvm-project"]
	path = src/llvm-project
	url = https://github.com/rust-lang/llvm-project.git
	branch = rustc/9.0-2019-09-19

[ctaggart]

I don't think this is required. Time will tell.