Handle the case where no eh_frame section is present.

RegisterContextLLDB holds a reference to the SymbolContext
in the vector of Cursors that UnwindLLDB maintains.  Switch
UnwindLLDB to hold a vector of shared pointers of Cursors
so this reference doesn't become invalid.

Correctly falling back from the "fast" UnwindPlan to the
"full" UnwindPlan when additional registers need to be
retrieved.

llvm-svn: 118218