Remove size_t return parameter from FindTypes

In r368345 I accidentally introduced a regression that would
over-report the number of matches found by FindTypes if the
DeclContext Filter was hit.

This patch simply removes the size_t return parameter altogether —
it's not that useful.

rdar://problem/55500457

Differential Revision: https://reviews.llvm.org/D68169

llvm-svn: 373344