clang-format: Fix corner case with comment in ctor initializer.

Formatting:
  Constructor() :
      // Comment forcing unwanted break.
      aaaa(aaaa) {}

Before:
  Constructor()
      :
        // Comment forcing unwanted break.
        aaaa(aaaa) {}

After:
  Constructor()
      : // Comment forcing unwanted break.
        aaaa(aaaa) {}

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199107 91177308-0d34-0410-b5e6-96231b3b80d8