Neither really.. It's a typographic symbol. Just like the Japanese period symbol (。) isn't hiragana or katakana or kanji, and just like the period symbol using in English (.) isn't part of the alphabet. ^^
That ー symbol is called 長音符(ちょうおんぷ), which should be enough to explain what it does. ^^
長(ちょう)"long"
音(おん)"sound"
符(ふ)"mark" https://en.wikipedia.org/wiki/Chouonpu