Fix non-determinism issue with implicit lambda captures.

We were using llvm::SmallPtrSet for our ODR-use set which was also used
for instantiating the implicit lambda captures. The order in which the
captures are added depends on this, so the lambda's layout ended up
changing.  The test just uses floats, but this was noticed with other
types as well.

This test replaces the short-lived SmallPtrSet (it lasts only for an
expression, which, though is a long time for lambdas, is at least not
forever) with a SmallSetVector.