Updates prereqs to build LLVM from source for C++17.

[red1bluelost]

With the recent upgrade to C++17 in LLVM, this increases the minimum requirements for building LLVM from source. Matches those at https://llvm.org/docs/GettingStarted.html#host-c-toolchain-both-compiler-and-standard-library

[tshepang]

do these minimums apply to older llvm versions, which we support

[red1bluelost]

Good point! LLVM upgraded on August 6 which was very recent. Rust's fork of LLVM is still using C++14 so the old compilers would still be viable.
Maybe this PR can just be blocked until Rust updates the fork to the C++17 LLVM.

[tshepang]

the text could be changed to link to pre-reqs pages of the supported llvm versions, which can be older than the fork

[red1bluelost]

I edited the text point to the recent LLVM 14 release documentation and added a date check comment. Does this work?

[tshepang]

it does, thanks

[tshepang]

I edited the text point to the recent LLVM 14 release documentation and added a date check comment.

I assume you meant LLVM 13

[red1bluelost]

Yes, oops my comment was wrong. The docs match regardless but I put 13 since it counts as one of the "last few" releases.

[tshepang]

it looks like the oldest one supported by nightly rust-lang/rust#100460