Fix SIL FileCheck tests.
4 remaining test failures:
```
Failing Tests (4):
Swift(linux-x86_64) :: AutoDiff/tbdgen.swift
Swift(linux-x86_64) :: AutoDiff/silgen_thunking/main.swift
Swift(linux-x86_64) :: AutoDiff/autodiff_indirect_diagnostics.swift
Swift(linux-x86_64) :: AutoDiff/class_method.swift
```
AutoDiff/silgen_thunking/main.swift is understood: `LinkEntity::getLinkage`
is not correct for SIL differentiability witnesses.
