Use cpp_namet's and namet's constructors

[tautschnig]

Constructing the underlying irept manually is error-prone. Also don't
reduandantly assert that a cpp_namet has id() == ID_cpp_name.

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[allredj]

Passed Diffblue compatibility checks (cbmc commit: 7f817ca).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/85718212