compiletest: error on duplicate directives

[jieyouxu]

E.g.

//@ add-minicore
//@ compile-flags: --target thumbv8m.main-none-eabi --crate-type lib
//@ needs-llvm-components: arm
//@ add-minicore

should produce an error.

However, note that this should only apply for non-additive directives, e.g.

//@ compile-flags: -Cabc
//@ compile-flags: -Cdef

are additive directives, so that must not error.

(Blocked on reworking how directives are registered/handled in the first place to avoid making more complications.)