[clang-format] Improve detection of ObjC for-in statements

Summary:
Previously, clang-format would detect the following as an
Objective-C for-in statement:

  for (int x = in.value(); ...) {}

because the logic only decided a for-loop was definitely *not*
an Objective-C for-in loop after it saw a semicolon or a colon.

To fix this, I delayed the decision of whether this was a for-in
statement until after we found the matching right-paren, at which
point we know if we've seen a semicolon or not.

Test Plan: New tests added. Ran tests with:
  make -j12 FormatTests && ./tools/clang/unittests/Format/FormatTests

Reviewers: krasimir, jolesiak

Reviewed By: jolesiak

Subscribers: djasper, cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D43904

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@326815 91177308-0d34-0410-b5e6-96231b3b80d8