Add a new SBProcess:: GetCoreFile() API (#80767)

We have a Python script that needs to locate coredump path during
debugging so that we can retrieve certain metadata files associated with
it. Currently, there is no API for this.

This patch adds a new `SBProcess::GetCoreFile()` to retrieve target dump
file spec used for dump debugging. Note: this is different from the main
executable module spec. To achieve this, the patch hoists m_core_file
into PostMortemProcess for sharing.

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Co-authored-by: jeffreytan81 <jeffreytan@fb.com>