Consistently prefix executables

Inspired by Sylvestre's version suffix proposal, I want to bring up my
past request to clean up the executable names in llvm's bin/.

Looking through a 6.0 install, I see the following files which
would ideally be prefixed with "ll" or similar:

FileCheck
bugpoint
c-index-test
count
find-all-symbols
modularize
not
obj2yaml
opt
sancov
sanstats
scan-build
scan-view
verify-uselistorder
wasm-ld
yaml-bench
yaml2obj

If I were a distro packager, I might have to rename or filter executables.