[clang-tidy] readability-redundant-declaration: fix false positive with C "extern inline"

Summary:
readability-redundant-declaration was diagnosing a redundant declaration
on "extern inline void f();", which is needed in C code to force an external definition
of the inline function f. (This is different to how inline behaves in C++).

Reviewers: alexfh, danielmarjamaki

Subscribers: xazax.hun, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61700

llvm-svn: 360613