[SEH] Fix regression with SEH in noexpect functions

Commit 5baea056 set the CurCodeDecl
because it was needed to pass the assert in CodeGenFunction::EmitLValueForLambdaField,
But this was not right to do as CodeGenFunction::FinishFunction passes it to EmitEndEHSpec
and cause corruption of the EHStack.

Revert the part of the commit that changes the CurCodeDecl, and instead
adjust the assert to check for a null CurCodeDecl.

Differential Revision: https://reviews.llvm.org/D102027