[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