[LCG] Fix an assert in a on-scope-exit lambda that checked the contents

[LCG] Fix an assert in a on-scope-exit lambda that checked the contents
of the returned value.

Checking the returned value from inside of a scoped exit isn't actually
valid. It happens to work when NRVO fires and the stars align, which
they reliably do with Clang but don't, for example, on MSVC builds.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310547 91177308-0d34-0410-b5e6-96231b3b80d8