Revert "PR17164: Change clang's default behavior from...

Revert "PR17164: Change clang's default behavior from -flax-vector-conversions=all to -flax-vector-conversions=integer."

This patch broke the Sanitizer buildbots. Please see the commit's
differential revision for more information
(https://reviews.llvm.org/D67678).

This reverts commit b72a8c65.