[autocompletion] Handle the space before pressing tab

Summary:
Distinguish "--autocomplete=-someflag" and "--autocomplete=-someflag,"
because the latter indicates that the user put a space before pushing tab
which should end up in a file completion.

Differential Revision: https://reviews.llvm.org/D53639

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@345133 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed