Rollup merge of #123881 - erickt:bump-fuchsia, r=tmandry

Bump Fuchsia versions

This updates the Fuchsia commit used in `auto - x86_64-gnu-integration` CI bot to use the Rust commit 703dc9c. This should help improve the coverage of this builder.

It also updates the SDK version to F20.20240412.3.1, and the Fuchsia Clang version to c777c011a709dffd4fa5e79cad7947b7c3405d02.

r? ``@tmandry``

Author: fmease