[gdb/symtab] Fix assert in process_imported_unit_die

[gdb/symtab] Fix assert in process_imported_unit_die

When running test-case gdb.dwarf2/dw2-symtab-includes.exp with target board
cc-with-debug-names, I run into:
...
(gdb) maint expand-symtab dw2-symtab-includes.h^M
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 binary reading speed optimization only works in the
normal use case.

IWBN to have this working in all use cases, but for now, allow lang () and
unit_type () to return language_unknown and 0 here.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29321