Investigate prefetching PE file data into pinned managed memory.

dotnet/roslyn#24939 describes a situation where creating `PEReader`s with `PEStreamOptions.PrefetchEntireImage` leads to OOMs due to large native allocations that the GC cannot keep track of.

I wonder if it would be better to instead allocate the prefetched data in managed memory and later pin it, akin to how `PEReader(ImmutableArray)` works. If so, we can make use of the Pinned Object Heap on modern .NET; the question is whether to keep using native memory on downlevel frameworks, or allocate and pin there as well.

Start by reading dotnet/roslyn#24939 and reviewing the PEReader(ImmutableArray) path alongside PEStreamOptions.PrefetchEntireImage. Compare managed allocation and pinning behavior with the Pinned Object Heap on modern .NET and downlevel frameworks. Done means establishing whether the proposed approach avoids the reported native-memory OOMs and documenting the allocation strategy decision.

Written by the indexing model from the issue text.