When running test-case gdb.dwarf2/dw2-symtab-includes.exp with target board cc-with-debug-names (same with target boards cc-with-gdb-index and readnow), I run into:
...
(gdb) maint expand-symtab dw2-symtab-includes.h^M
/home/vries/gdb_versions/devel/src/gdb/dwarf2/read.h:311: internal-error: unit_type: Assertion `m_unit_type != 0' failed.^M
A problem internal to GDB has been detected,^M
further debugging may prove unreliable.^M
----- Backtrace -----^M
FAIL: gdb.dwarf2/dw2-symtab-includes.exp: maint expand-symtab dw2-symtab-includes.h (GDB internal error)
...

The assert was recently added in commit 2c474c46943 ("[gdb/symtab] Add get/set functions for per_cu->lang/unit_type").

The assert is triggered here:
...
      /* We're importing a C++ compilation unit with tag DW_TAG_compile_unit          
         into another compilation unit, at root level.  Regard this as a hint,        
         and ignore it.  */
      if (die->parent && die->parent->parent == NULL
          && per_cu->unit_type () == DW_UT_compile
          && per_cu->lang () == language_cplus)
        return;
...

We're trying to access unit_type / lang which hasn't been set yet.

Normally, these are set during cooked index creation, but that's not the case when using an index (or using -readnow).

In other words, this lto speed optimization only worked in the normal usage case.