Fix GC hole in string marshaling

[filipnavara]

Fixes #768

Fix GC hole where the whole MarshalString object gets moved and makes the HSTRING reference invalid
Fix memory leak and possible memory corruption in exception handling in GetActivationFactory

The implementation of WindowsCreateStringReference takes the header buffer, fills it and then the returned handle is a pointer to the header buffer. It is thus absolutely essential that the header buffer is not moved while the HSTRING instance is alive. This is explicitly called out in the documentation:

The caller must ensure that sourceString and the contents of hstringHeader remain unchanged during the lifetime of the attached HSTRING.

The code was storing the header buffer in unpinned GC memory so the GC was free to move it and it occasionally did so which caused accesses from native code into invalid memory.

Now the solution in the PR is not the most optimal one but it's one that is least disruptive and easy to review. From performance point of view it would make sense to use ref struct and rely on the stack being unmovable.

[filipnavara]

@angelazhangmsft Please give this issue/PR appropriate attention. It's serious enough that it makes CsWinRT totally unusable for us due to random crashes. I am not pushing for accepting my fix verbatim but it's likely affecting all CsWinRT users to certain extent.

[Scottj1s]

@filipnavara - really appreciate this fix. yes, GC moves had not been considered.