Replace an assert for a CXXCtorInitializer SourceLocation with a
condition, as was done for other locations in commit 54272e5b (NFC:
Replace asserts with if() in SourceLocation accessors, 2019-01-07).
Fix the logic of detecting pseudo-virtual getBeginLoc etc on Stmt and
Decl subclasses.
Adjust the test infrastructure to filter out invalid source locations.
This makes the tests more clear about which nodes have which locations.