Merging r330926:

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r330926 | sepavloff | 2018-04-25 23:28:47 -0700 (Wed, 25 Apr 2018) | 9 lines

[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).

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llvm-svn: 332649