Don't trim IMarshal

[ericstj]

The linker was trimming out IMarshal because it saw the interface was never used by managed code.

This does get used by COM and the interface being present but empty breaks native code when it attempts to call an method that is missing from the interface.

/cc @luqunl @Tanya-Solyanik

[ericstj]

It strikes me that we'd probably have discovered this if we had some simple testcases to test the functionality of StandardOleMarshal. I can see about adding those.

[danmoseley]

I agree a test would probably be wise

[jkotas]

Do we need to actually fix the linker to not trim [ComImport] interfaces? There are number of other [ComInterfaces] used throughout the CoreFX that may be hitting similar issue.

cc @sbomer

[ericstj]

@jkotas, yes, I'm typing up that issue now. Also I did a new pass at looking at the linker diffs to find more of these. Working on uploading those reports to azure so I can link them in the deadcode issue.

[ericstj]

To circle back, I reviewed all the [ComImport] interfaces and found that this appears to be the only one trimmed.

We have a lot in S.R.WindowsRuntime which will get wacked if I force trimming on that assembly, but we're already opting out due to this:

corefx/illink.targets

Lines 56 to 57 in 0b0f63d

	<!-- Currently ILLink cannot handle type projections from Windows.winmd, disable if the project references it -->
	<ILLinkTrimAssembly Condition="'%(ReferencePath.FileName)%(ReferencePath.Extension)' == 'Windows.winmd'">false</ILLinkTrimAssembly>