[SYCL] Fix invalid use of std::move in SemaTemplateInstantiateDecl.cpp

[erichkeane]

This being used in a loop with more than one multiversion function would likely cause that mangle-context to be invalid. Also, the unqualified move is discouraged.

This patch just changes the code slightly to have the lambda capture the entire unique_ptr by reference.

[elizabethandrews]

This doesn't look like a SYCL specific change. So is there a reason this is not being done in community clang instead? Also, is there a test you can add since this isn't a NFC. I assume this is fixing a crash/mis-compilation

[erichkeane]

This doesn't look like a SYCL specific change. So is there a reason this is not being done in community clang instead? Also, is there a test you can add since this isn't a NFC. I assume this is fixing a crash/mis-compilation

This IS a SYCL specific change, this code (processFunctionInstantiation) doesn't exist in the upstream.

I don't have a reproducer for the crash. READING the code, I'd expect it should, but it doesn't (Aaron and I both read it and are surprised). There are tests that SHOULD run into this, but no idea why they dont. My best guess is the host compiler doing us favors with inlining. The motivation for the change is the unqualified use of 'move', which I couldn't in good conscience JUST qualify when it was so broken. (that is, make move into std::move);