ValueMapper: Disallow metadata mapping recursion through mapValue

ValueMapper: Disallow metadata mapping recursion through mapValue

This adds an assertion to maintain the property from r265273.  When
Mapper::mapSimpleMetadata calls Mapper::mapValue, it should not find its
way back to mapMetadataImpl.  This guarantees that mapSimpleMetadata is
not involved in any recursion.

Since Mapper::mapValue calls out to arbitrary materializers, we need to
save a bit on the ValueMap to make this assertion effective.

There should be no functionality change here.  This co-recursion should
already have been impossible.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@265276 91177308-0d34-0410-b5e6-96231b3b80d8