clang-format: Add spaces around trailing/lambda return types.

Before:
  int c = []()->int { return 2; }();

After:
  int c = []() -> int { return 2; }();

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203452 91177308-0d34-0410-b5e6-96231b3b80d8