MemorySanitizer has no visibility into writes performed by JIT'd code.

If we have built the project with the MemorySanitizer enabled, memset the Triangle and Primitive structs so the sanitizer has a write to see.

This is only cleared in MemorySanitizer enabled builds, as this adds a non negligable cost to Renderer construction.

Bug: chromium:998457
Change-Id: I3b4f9e48783c6e8a1109a578988567b0a42eb1f9
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/35688
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>