[clang] Implement Override Suggestions in Sema.

[clang] Implement Override Suggestions in Sema.

Summary:
In clangd we had a new type of completion suggestions for cpp
class/struct/unions that will show override signatures for virtual methods in
base classes. This patch implements it in sema because it is hard to deduce more
info about completion token outside of Sema and handle itchy cases.

See the patch D50898 for more info on the functionality.

In addition to above patch this one also converts the suggestion into a
CK_Pattern with whole insertion text as the name of the suggestion and factors
out CodeCompletionString generation for declerations so that it can be re-used
by others.

Reviewers: ioeric, ilya-biryukov

Reviewed By: ioeric

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D52225

llvm-svn: 343568