Minor cleanups for install_llvm_module/uninstall_llvm_module Make macros.

Minor cleanups for install_llvm_module/uninstall_llvm_module Make macros.

Don't put comments inside the macros, per complaint from Michael Paquier.

Quote target directory path with single quotes, not double; that seems
to be our project standard.  Not quoting it at all definitely isn't
per standard.

Remove excess slash in one instance of path.

Remove useless semicolon.

Discussion: https://postgr.es/m/20180428073935.GB1736@paquier.xyz