[lldb] Fix assertion caused by invalid SupportFileSP (llvm#162710)

SupportFileSP should never be null, and instead should use a default
constructed SupportFile to represent an invalid instance. This is
because the class used to be a value type before it became polymorphic.

We have various places in LLDB where we check this precondition,
including in DisplaySourceLinesWithLineNumbers. The assertion was
tripped when calling GetStartLineSourceInfo which starts by resetting
the SupportFileSP and has a series of early returns which leave the
shared pointer in that state.

rdar://161607247

Author: JDevlieghere

lldb/source/Symbol/Function.cpp

@@ -275,7 +275,7 @@ Function::~Function() = default;
 void Function::GetStartLineSourceInfo(SupportFileSP &source_file_sp,
                                       uint32_t &line_no) {
   line_no = 0;
-  source_file_sp.reset();
+  source_file_sp = std::make_shared<SupportFile>();
 
   if (m_comp_unit == nullptr)
     return;