Define `MemoryKind::ByrefExposed`

Define `MemoryKind::ByrefExposed`

Add a new `MemoryKind` to represent "any memory that byrefs may
reference", called `ByrefExposed`.  All definition points of `GcHeap`
become also definition points of `ByrefExposed`, and additionally, so do
definitions of address-exposed locals.

Because it is a common case (currently happening in 90% of methods in
System.Private.CoreLib) that a method has no definitions of
address-exposed locals, have liveness detect this condition and set the
new `byrefStatesMatchGcHeapStates` flag accordingly; when the states
match, the same `MemoryPhi`s, defnums, and `PerSsaData` are then shared
between the two memory kinds, to avoid excess allocations/processing.

This change defines `ByrefExposed` memory and its def/use points, and
builds SSA for it, but no optimizations make use of it, so this is a
no-diff change.

Commit migrated from https://github.com/dotnet/coreclr/commit/1c6a419710ed292f2fa48e459239ee95e3ed2ca2