IR: Fix a race condition in type id clients of ModuleSummaryIndex.

Add a const version of the getTypeIdSummary accessor that avoids
mutating the TypeIdMap.

Differential Revision: https://reviews.llvm.org/D31226

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298531 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed