Don't require non-class types to be complete to convert from them (#7590)

In Convert, we require the source value's type to be compete so that we
can look for `base` classes and `adapt` relationships. However these can
only be present in a `ClassType`, so we only need `ClassType`s to be
complete.

Reduce the requirement in Convert to not complete types that are not a
`ClassType`, and which can not contain a `ClassType` as part of their
class.

Ideally we would only _only_ require the `ClassType` itself to be
complete, and only if we're looking for a base or adapt. However lower
depends on us completing all Convert source types that contain a class.
This seems to suggest we're lacking checks for complete types somewhere
else and Convert is making up for it. A TODO has been added. The
`toolchain/driver/testdata/compile/optimize/optimize_debug.carbon` test
is an example that CHECKs due to failing to verify the LLVM module if we
do not compute the complete type of all class-containing types in
Convert.

The critical step this PR is doing is to stop trying to complete a
`FacetType` when converting from a facet. This avoids trying to complete
a named constraint when converting `Self` inside that named constraint.
Doing so causes a cycle when the conversion of `Self` is performed in
eval of an `extend require` decl, since requiring the named constraint
to be complete re-evaluates the `extend require` decl again. A test is
added that crashed in an infinite loop before this change.

It also depends on #7584, which was intended to be an optimization but
is now load bearing. Because converting `Self` leaves an impl lookup
inst behind, and if that inst is re-evaluated inside impl lookup (by
forming a specific of a `require` decl through identify) then we have a
similar cycle.

toolchain/check/testdata/interop/cpp/function/import/overloads.carbon

@@ -4069,9 +4069,7 @@ fn F() {
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: specific @I.WithSelf(constants.%Self) {
-// CHECK:STDOUT: !definition:
-// CHECK:STDOUT: }
+// CHECK:STDOUT: specific @I.WithSelf(constants.%Self) {}
 // CHECK:STDOUT:
 // CHECK:STDOUT: specific @EchoValue(constants.%ValueT, constants.%value) {
 // CHECK:STDOUT:   %ValueT.patt.loc10_20.2 => constants.%ValueT.patt