[LLDB][ClangExpression] Prevent nullptr namespace map access during logging

Some codepaths lead to `namespace_map == nullptr` when we get to
`ClangASTSource::FindCompleteType`. This occurred while debugging
an lldb session that had `settings set target.import-std-module true`.

In that case, with `LLDBLog::Expressions` logging enabled, we would
dereference a `nullptr` and crash.

This commit moves the logging until after we check for `nullptr`.

**Testing**

* Fixed the specific crash I was seeing while debugging an `lldb`
  session with `import-std-module` enabled.

Differential Revision: https://reviews.llvm.org/D130561