devel/llvm*: LICENSE_DIR simplification

WRKSRC moved to the top of the source tree with llvm12. As a result,
LICENSE_DIR became ${WRKSRC}/llvm. Given that llvm11 the only port that
wasn't setting LICENSE_DIR, flip the script and default to
${WRKSRC}/llvm with llvm11 overriding.

Sponsored by: DARPA, AFRL