[Basic] Move DiagnosticsEngine::dump from .h to .cpp

The two LLVM_DUMP_METHOD methods have a undefined reference on clang::DiagnosticsEngine::DiagStateMap::dump.

tools/clang/tools/extra/clangd/benchmarks/IndexBenchmark links in
clangDaemon but does not link in clangBasic explicitly, which causes a
linker error "undefined symbol" in !NDEBUG + -DBUILD_SHARED_LIBS=on builds.

Move LLVM_DUMP_METHOD methods to .cpp to fix IndexBenchmark. They should
be unconditionally defined as they are also used by non-dump-method #pragma clang __debug diag_mapping

llvm-svn: 348065