Re-enable -Wshadow -Werror=shadow

[JJL772]

There's an edge case with the STATIC_ASSERT macro provided by EPICS when building with C++03 and earlier. Specifically when STATIC_ASSERT appears in global scope in an included file on the same line number as a function-scope STATIC_ASSERT in a different file.

PR: epics-base/epics-base#401

We'll probably never be able to enable -Werror=shadow in normal builds again, for compatibility reasons. But for BUILD_STRICT in CI, that's fair game after the aforementioned fix is merged.