Enhance -Wshadow to warn when shadowing typedefs or type aliases

Enhance -Wshadow to emit a warning when typedefs or type aliases are shadowed.

Fixes bug https://bugs.llvm.org//show_bug.cgi?id=28676.

Patch by Ahmed Asadi.

Differential Revision: https://reviews.llvm.org/D31235

llvm-svn: 299363