[ThinLTOCodeGenerator] Avoid Rehash StringMap in ThreadPool

Summary:
During threaded thinLTO, it is possible that the entry for current
module doesn't exist in StringMaps (like ExportLists, ResolvedODR,
etc.). Using operator[] might trigger a rehash for the StringMap, which
might happen on multiple threads at the same time.

rdar://problem/43846199

Reviewers: tejohnson, mehdi_amini, kromanova, pcc

Reviewed By: tejohnson

Subscribers: dang, inglorion, eraman, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D52049

llvm-svn: 342263