Merging r352119:

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r352119 | rnk | 2019-01-24 23:26:51 +0100 (Thu, 24 Jan 2019) | 7 lines

[clang-cl] Ignore space-separated /AI arguments

The /AI flag is for #using directives, which I don't think we support.
This is consistent with how the /I flag is handled by MSVC.  Add a test
for it.

Differential Revision: https://reviews.llvm.org/D57189
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llvm-svn: 355675