[clang-format] Always break line after enum opening brace

Summary:
clang-format currently puts the first enumerator on the same line as the
enum keyword and opening brace if it fits (for example, for anonymous
enums if IndentWidth is 8):

  $ echo "enum { RED, GREEN, BLUE };" | clang-format -style="{BasedOnStyle: llvm, ColumnLimit: 15, IndentWidth: 8}"
  enum { RED,
         GREEN,
         BLUE };

This doesn't seem to be intentional, as I can't find any style guide that
suggests wrapping enums this way. Always force the enumerator to be on a new
line, which gets us the desired result:

  $ echo "enum { RED, GREEN, BLUE };" | ./bin/clang-format -style="{BasedOnStyle: llvm, ColumnLimit: 15, IndentWidth: 8}"
  enum {
          RED,
          GREEN,
          BLUE
  };

Test Plan:

New test added. Confirmed test failed without change and passed with change by
running:

  $ ninja FormatTests && ./tools/clang/unittests/Format/FormatTests

Reviewed By: MyDeveloperDay

Patch By: osandov

Tags: #clang-format, #clang

Differential Revision: https://reviews.llvm.org/D77682