A recent SwS roll changed the deviceName of swiftshader to include the
backend (LLVM for example).  That made ANGLE no longer recognize
SwiftShader.  This change looks for the known SwS device name as a
prefix of the deviceName retrieved from the device, rather than a
complete match.

Bug: angleproject:3876
Change-Id: If6dacb1e5685493d5334cad2015439f0f6b71124
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1980275Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>