Fix step-over when SymbolContext.function is missing and symbol is present.

Summary:
Fix step-over when SymbolContext.function is missing and symbol is present.

With targets from our build configuration,
ThreadPlanStepOverRange::IsEquivalentContext fails to fire for relevant frames,
leading to ShouldStop() returning true prematurely.

The frame's SymbolContext, and m_addr_context have:
  - comp_unit set and matching
  - function = nullptr
  - symbol set and matching (but this is never checked)
My naive guess is that the context should be equivalent in this case :-)

Reviewers: jingham

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D26804

llvm-svn: 287274