Fix formatting of preprocessor directives (incluces, warnings & errors).

Treat tokens inside <> for includes and everything from the second token
of a warning / error on as an implicit string literal, e.g. do not
change its whitespace at all.

Now correctly formats:
 #include < path with space >
 #error Leave     all         white!!!!! space* alone!

Note that for #error and #warning we still format the space up to the
first token of the text, so:
   #  error   Text
will become
 #error Text

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172536 91177308-0d34-0410-b5e6-96231b3b80d8