[JSONNodeDumper] Do not print mangled names for local variables (PR49111)

Mangled names are not meaningful for variables with local storage,
and may not be well defined (getting the mangled name for VLA
crashes the mangler). As such, do not include them in the JSON
dump.

This allows running update_cc_test_checks on some OpenMP tests again.

Fixes https://github.com/llvm/llvm-project/issues/49111.

Differential Revision: https://reviews.llvm.org/D116169