Add the ability for an SBValue to create a persisted version of itself.

Such a persisted version is equivalent to evaluating the value via the expression evaluator, and holding on to the $n result of the expression, except this API can be used on SBValues that do not obviously come from an expression (e.g. are the result of a memory lookup)

Expose this via SBValue::Persist() in our public API layer, and ValueObject::Persist() in the lldb_private layer

Includes testcase

Fixes rdar://19136664

llvm-svn: 223711