Backport change to fix build on ToT Clang

This backports upstream change:
https://quiche.googlesource.com/quiche/+/c1af894e0f5c4f732a983e7c93227854e203570e

    Fix QuicIntervalDeque::Iterator::operator+=

    The code was referring to a member which didn't exist. This was uncovered by a
    recent Clang change [1] which made the compiler lookup these names at template
    parse time rather than instantiation time.

     1. https://github.com/llvm/llvm-project/pull/90152

    PiperOrigin-RevId: 630952202

Test: presubmit
Change-Id: I82121b22326dd0a5b0db65463b11f174b6aa81fe
1 file changed