fix a minor bug I noticed while work with Jordy's patch for PR6101,

fix a minor bug I noticed while work with Jordy's patch for PR6101,
in an input file like this:

# 42
int x;

we were emitting:

# <something>
 int x;

(with a space before the int) because we weren't clearing the leading
whitespace flag properly after the \n from the directive was handled.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@101084 91177308-0d34-0410-b5e6-96231b3b80d8