LLVM: rustllvm produces unsilenceable warnings on new versions of GCC (9.2.0)

[petrochenkov]

... which are pretty annoying.
It probably wouldn't be a problem if Cargo didn't cache the warnings and didn't display them on every build, even if rustllvm isn't actually built.

Example of the warnings:

warning: In file included from ../rustllvm/rustllvm.h:4,
warning:                  from ../rustllvm/PassWrapper.cpp:6:
warning: C:/msys64/home/we/rust/src/llvm-project/llvm/include/llvm/ADT/ArrayRef.h: In instantiation of 'llvm::ArrayRef<T>::ArrayRef(const std::initializer_list<_Tp>&) [with T = long long unsigned int]':
warning: C:/msys64/home/we/rust/src/llvm-project/llvm/include/llvm/IR/DIBuilder.h:644:74:   required from here
warning: C:/msys64/home/we/rust/src/llvm-project/llvm/include/llvm/ADT/ArrayRef.h:101:37: warning: initializing 'llvm::ArrayRef<long long unsigned int>::Data' from 'std::initializer_list<long long unsigned int>::begin' does not extend the lifetime of the underlying array [-Winit-list-lifetime]
warning:   101 |     : Data(Vec.begin() == Vec.end() ? (T*)nullptr : Vec.begin()),

This is fixed in LLVM upstream (llvm/llvm-project@6c2151b), so that commit can be cherry-picked into our LLVM fork.

The commit seems to be included into LLVM 10, but I'm not sure when #67759 will be able to land.

[Mark-Simulacrum]

We might want to hide the LLVM/rustllvm build log from users -- similar to how Cargo hides upstream, non-local, path deps. We'd probably want to stick this behind an option or something like that for those who are compiling LLVM after changing it locally.