[clang][DebugInfo] Don't canonicalize names in template argument list for alias templates

**Summary**

This patch customizes the `CGDebugInfo` printing policy to stop canonicalizing
the template arugment list in `DW_AT_name` for alias templates. The motivation for
this is that we want to be able to use the `TypePrinter`s support for
omitting defaulted template arguments when emitting `DW_AT_name`.

For reference, GCC currently completely omits the template arguments
when emitting alias template DIEs.

**Testing**

* Added unit-test

Differential Revision: https://reviews.llvm.org/D142268