[clang-tidy] Don't emit misc-unused-using-decl warnings for header files.

Using decls in header files are special, usually as part of the
public API, the check should not emit warnings on these.

The check already detects unused using-decls which are in the current main
file, but if the main file happens to be a header file, we still
emit warnings, this patch suppresses that.

Differential Revision: https://reviews.llvm.org/D140894