[ThinLTO] Add string saver onto index for value names

[ThinLTO] Add string saver onto index for value names

Summary:
Adds a string saver to the ModuleSummaryIndex so it can store value
names in the case of adding a ValueInfo for a GUID when we don't
have the name stored in a Module string table. This is motivated
by the upcoming summary parser patch, where we will read value names
from the summary entry and want to store them, even when a Module
is not available.

Currently this allows us to store the name in the legacy bitcode case,
and I have added a test to show that.

Reviewers: pcc, dexonsmith

Subscribers: mehdi_amini, inglorion, eraman, steven_wu, llvm-commits

Differential Revision: https://reviews.llvm.org/D47842

llvm-svn: 335570