[clang-repl] Extend the C support. (#89804)

The IdResolver chain is the main way for C to implement lookup rules.
Every new partial translation unit caused clang to exit the top-most
scope which in turn cleaned up the IdResolver chain. That was not an
issue for C++ because its lookup is implemented on the level of
declaration contexts.

This patch keeps the IdResolver chain across partial translation units
maintaining proper C-style lookup infrastructure.