nvc: go back to using default llvm as dependency
nvc: go back to using default llvm as dependency

The restriction on building against llvm@6 (later updated to llvm@7)
was introduced by @jnozsc (#54741)  However the issue that was
referenced was fixed before 1.5.0 was released...
https://github.com/nickg/nvc/commit/f48c651eeb08ba273610fff57c71d24990ec7c34
...so when homebrew updated to 1.5.0 this restriction should probably
be dropped.

According to the project's README it is tested with llvm {7,8,9} so
at a minimum we should probably bump its dependency to llvm@9.  However,
this wouldn't be enough to fix the Big Sur bottling issue.

Nothing in the project's build instructions suggests that llvm versions
newer than 9.x are to be avoided these days so just switching to our
default llvm (currently 11.1) instead.  Seems to build and test fine.

Closes #71444.

Signed-off-by: Carlo Cabrera <3ffc397d0e4bded29cb84b56167de54c01e3a55b@users.noreply.github.com>
Signed-off-by: BrewTestBot <8a898ee6867e4f2028e63d2a6319b2224641c06c@users.noreply.github.com>

nandub/homebrew-core,stgraber/homebrew-core,uyjulian/homebrew-core,Homebrew/homebrew-core,Linuxbrew/homebrew-core,Moisan/homebrew-core,adam-moss/homebrew-core,nandub/homebrew-core,spaam/homebrew-core,mbcoguno/homebrew-core,zyedidia/homebrew-core,Linuxbrew/homebrew-core,lemzwerg/homebrew-core,zyedidia/homebrew-core,lemzwerg/homebrew-core,sjackman/homebrew-core,chrisfinazzo/homebrew-core,cblecker/homebrew-core,adam-moss/homebrew-core,JCount/homebrew-core,makigumo/homebrew-core,makigumo/homebrew-core,chrisfinazzo/homebrew-core,stgraber/homebrew-core,mbcoguno/homebrew-core,sjackman/homebrew-core,filcab/homebrew-core,Homebrew/homebrew-core,mfikes/homebrew-core,uyjulian/homebrew-core,spaam/homebrew-core,JCount/homebrew-core,nbari/homebrew-core,cblecker/homebrew-core,mbcoguno/homebrew-core,Moisan/homebrew-core,nbari/homebrew-core,mfikes/homebrew-core,filcab/homebrew-core