Generate the full debug info for generic structs. The strategy is to
emit one full entry for the generic type with archetypes, and one
forward declaration per instantiation of the generic type.
For example, given:
```
struct Pair<T, U> {
let t: T
let u: U
}
let p1 = Pair<Int, Double>(t: 1, u: 4.2)
let p2 = Pair<String, Bool>(t: "Hello", u: true)
```
DebugInfo will have one entry for Pair<T, U> which includes descriptions
of its fields, one forward declaration of Pair<Int, Double> and one
forward declaration of Pair<String, Bool>. This information is enough
for the algorithms of RemoteMirrors to reconstruct type information for
the fully instantiated types.
Previously type sizes would be inconsistently sourced from either the LLVM type
or the FixedTypeInfo, depending on the call site. This was problematic because
TypeInfo operates with a resolution of whole bytes, which means that types such
as i1 would get a reported as having a size of 8. This patch now asserts that
all occurrences of the same type have the same size as the first, cached
occurence.
To avoid triggering the cached type verification assertion, this patch avoids
caching of storage-sized containers. It also removes the unique identifier from
forward declarations, which could lead to type confusion during LTO.
rdar://102367872
This is needed to anchor any typedefs that appear in bound generic parameters so
they can be resolved in the typeref-based part of the debugger without needing
to query the Swift module.
