clang-format: Fix formatting C++ namespaces with preceding 'inline' or 'export' specifier

This fixes formatting namespaces with preceding 'inline' and 'export' (Modules TS) specifiers.

This change fixes namespaces not being identified as such with preceding 'inline' or 'export' specifiers.

Motivation: I was experimenting with the Modules TS (-fmodules-ts) and found it would be useful if clang-format would correctly format 'export namespace'. While making the changes, I noticed that similar issues still exist with 'inline namespace', and addressed them as well.

Patch by Marco Elver!

Reviewers: klimek, djasper, owenpan, sammccall

Reviewed By: owenpan, sammccall

Subscribers: owenpan, cfe-commits

Differential Revision: https://reviews.llvm.org/D51036

llvm-svn: 341450