clang-format: Understand proto text format without commas.

Also removed spaces before colons as they don't seem to be used
frequently.

Before:
optional int32 b = 2
    [(foo_options) = {aaaaaaaaaaaaaaaaaaa : 123 bbbbbbbbbbbbbbbbbbbbbbbb :
                          "baz"}];

After:
optional int32 b = 2 [(foo_options) = {aaaaaaaaaaaaaaaaaaa: 123,
                                       bbbbbbbbbbbbbbbbbbbbbbbb:"baz"}];

llvm-svn: 206269