Fix a GCC 10 build failure.
Fix a GCC 10 build failure.

Build failed on GCC10 with C++17. An #ifdef was added to support both
C++17 and C++14

Bug: angleproject:4390
Change-Id: I397513a8ebad4a55635ba863c47edc658e405729
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2063545
Commit-Queue: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle