vulkan: Add support for non-fatal missing functions in vulkan loader

When a function is added by a later revision of an extension, and the
loader expects to deal with drivers written against revisions before and
after that point, we need to not fail hard on missing that function.

Change-Id: Iac6383f6a424afe6e7f83acc86b674669a118c51
Test: Run Vulkan-LoaderAndValidationLayers Cube demo