[Chapel Merge] Documentation changes for CC and LLVM_CONFIG setti

Branch: refs/heads/main
Revision: 7a31068
Author: mppf
Log Message:

Merge pull request #17931 from mppf/docs-17923

Documentation changes for CC and LLVM_CONFIG settings

This PR contains the documentation changes to go along with PR #17923.

Reviewed by @daviditen - thanks!

Modified Files:
M doc/rst/usingchapel/chplenv.rst

M doc/rst/usingchapel/prereqs.rst
M third-party/llvm/README

Compare: https://github.com/chapel-lang/chapel/compare/18519a0c457e...7a310689edae