clang-format: Extend AllowShortFunctions.. to only merge inline functions.

Before AllowShortFunctionsOnASingleLine could either be true, merging
all functions, or false, merging no functions. This patch adds a third
value "Inline", which can be used to only merge short functions defined
inline in a class, i.e.:

  void f() {
    return 42;
  }

  class C {
    void f() { return 42; }
  };