change the string representation of xhp class and xhp attribute

Summary:
xhp class :x:y will have string form "<x:y>"
xhp attribute xstr will have string form "xstr="
This will fix the bug that :WebPage and WebPage are considered the same class

Test Plan:
eyeball + pass all tests
Codegraph on www will not have DUPE WebPage error

Reviewers: pad, mathieubaudet, pieter

CC: platform-diffs@lists

Differential Revision: https://phabricator.fb.com/D863739