Relax some preconditions for using FixedCompilationDatabase.

FixedCompilationDatabase (FCD) requires that the arguments it consumes after
'--' must not include positional parameters or the argv[0] of the tool. This
patch relaxes those restrictions.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194968 91177308-0d34-0410-b5e6-96231b3b80d8