[ConfigFiles] Update argument strings when merging argrument lists

Implementation of `InputArgList` assumes its field `ArgStrings` contains
strings for each argument exactly in the same order. This condition was
broken when arguments from config file and from invocation were merged.

This change fixes https://bugs.llvm.org/show_bug.cgi?id=37196 (Clang
config files can crash argument handling).

llvm-svn: 330926