Make the diagnostic-missing-prototypes put the suggested `static` in front of `const` if exists.

Summary:
Consider: `const int* get_foo() {return nullptr;}`
The suggested fix should be `static const int* get_foo(){}`
and not `const static int* get_foo(){}`

Reviewers: gribozavr2

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81444