[clang-tidy] Add modernize-concat-nested-namespaces check

Summary:
Finds instances of namespaces concatenated using explicit syntax, such as `namespace a { namespace b { [...] }}` and offers fix to glue it to `namespace a::b { [...] }`.

Properly handles `inline` and unnamed namespaces. ~~Also, detects empty blocks in nested namespaces and offers to remove them.~~

Test with common use cases included.
I ran the check against entire llvm repository. Except for expected `nested namespace definitions only available with -std=c++17 or -std=gnu++17` warnings I noticed no issues when the check was performed.

Example:
```
namespace a { namespace b {
void test();
}}

```
can become
```
namespace a::b {
void test();
}
```

Patch by wgml!

Reviewers: alexfh, aaron.ballman, hokein

Reviewed By: aaron.ballman

Subscribers: JonasToth, Eugene.Zelenko, lebedev.ri, mgorny, xazax.hun, cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D52136

llvm-svn: 343000