Fix SourceFile Visitor

[mattt]

As described in #16, #25, and #35, running swift-doc in a GitHub Action produces strange results, with overly-long API symbol names. While I'd originally attributed my inability to reproduce these results to a macOS v. Linux difference, it turns out to have been a matter of debug v. release.

I verified this locally by running tests in release mode (with the small change of making the import of SwiftDoc non-@testable):

$ swift test -c release
Test Suite 'All tests' failed at 2020-04-05 06:13:26.899.
	 Executed 1 test, with 12 failures (0 unexpected) in 0.072 (0.073) seconds

When building with a release configuration, the Swift compiler removes assertions. The problem with the current implementation is that we're doing something important in the assertion:

swift-doc/Sources/SwiftDoc/SourceFile.swift

Line 190 in 85b5872

assert((pop() as? Symbol)?.api is Class)

Separating out the work from the test in 7f9680e fixed the underlying issue.

To prevent such behavioral differences from creeping up again in the future, I updated the CI action to test in release mode going forward with 7896787. Tests will run slower than before, but this will give us greater confidence in the results of the tests, since it matches the configuration of how swift-doc is built when running make install.