This member is for some reason initialized in ClangASTSource::FindExternalVisibleDecls
so all other functions using this member dereference a nullptr unless we
call this function before that. Let's just initialize this in the constructor.
This should be NFC as the only side effect is that we don't reset the namespace map
when calling ClangASTSource::FindExternalVisibleDecls multiple times (and we never
call this function multiple times for one NameSearchContext from what I can see).
The size of NameSearchContext isn't important as we never store it and rarely
allocate more than a few. This way we also don't have to use the memset to
initialize these fields to zero.
Raphael Isemann