Give external linkage and mangling to lambdas inside inline variables and variable...

Give external linkage and mangling to lambdas inside inline variables and variable templates.

This implements the proposed approach in https://github.com/itanium-cxx-abi/cxx-abi/issues/33

This reinstates r313827, reverted in r313856, with a fix for the 'out-of-bounds
enumeration value' ubsan error in that change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@313955 91177308-0d34-0410-b5e6-96231b3b80d8