[clang][cli] Fix round-trip of OPT_plugin_arg

The test Frontend/plugin-delayed-template.cpp is failing when asserts
are enabled because it hits an assertion in denormalizeStringImpl when
trying to round-trip OPT_plugin_arg. Fix this by adjusting how the
option is handled, as the first part is joined to -plugin-arg and the
second is separate.

Differential Revision: https://reviews.llvm.org/D99606