compiletest: separate splitting name-value pairs versus name-value directives

[jieyouxu]

This is specifically for debugger checks, like

// cdb-check: xxx

this currently also uses parse_name_value_directive, but it's not a directive, and the check is actually handled by LLVM FileCheck, so the // vs //@ prefix does not matter.

Reason being that we want to make sure it's possible to know exhaustively what compiletest should consider a "known" directive.