[clang][CodeComplete] Propogate printing policy to FunctionDecl

Summary:
Printing policy was not propogated to functiondecls when creating a
completion string which resulted in canonical template parameters like
`foo<type-parameter-0-0>`. This patch propogates printing policy to those as
well.

Fixes https://github.com/clangd/clangd/issues/76

Reviewers: ilya-biryukov

Subscribers: jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72715