[modules] When either redecl chain merging or an update record causes us to

[modules] When either redecl chain merging or an update record causes us to
give an exception specification to a declaration that didn't have an exception
specification in any of our imported modules, emit an update record ourselves.
Without this, code importing the current module would not see an exception
specification that we could see and might have relied on.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@232870 91177308-0d34-0410-b5e6-96231b3b80d8