[ASTImporter] Add support for importing fixed point literals

Summary:
This patch adds support for importing fixed point literals, following
up to https://reviews.llvm.org/D46915 specifically for importing AST.

Reviewers: martong, leonardchan, ebevhan, a.sidorin, shafik

Reviewed By: martong

Subscribers: balazske, rnkovacs, teemperor, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77721