Fix jump on uninit producer_is_clang bit of cu.h dwarf2_cu struct.

Fix jump on uninit producer_is_clang bit of cu.h dwarf2_cu struct.

Valgrind reports a "jump on unitialised bit error" when running
    e.g. gdb.base/macro-source-path.exp (see details below).

    Fix this by initializing producer_is_clang member variable of dwarf2_cu.

    Tested on amd64/debian11 and re-running gdb.base/macro-source-path.exp
    under valgrind.

    ==2140965== Conditional jump or move depends on uninitialised value(s)
    ==2140965==    at 0x5211F7: dwarf_decode_macro_bytes(dwarf2_per_objfile*, buildsym_compunit*, bfd*, unsigned char const*, unsigned char const*, macro_source_file*, line_header const*, dwarf2_section_info const*, int, int, unsigned int, dwarf2_section_info*, dwarf2_section_info*, gdb::optional<unsigned long>, htab*, dwarf2_cu*) (macro.c:676)
    ==2140965==    by 0x52158A: dwarf_decode_macros(dwarf2_per_objfile*, buildsym_compunit*, dwarf2_section_info const*, line_header const*, unsigned int, unsigned int, dwarf2_section_info*, dwarf2_section_info*, gdb::optional<unsigned long>, int, dwarf2_cu*) (macro.c:967)
    ==2140965==    by 0x523BC4: dwarf_decode_macros(dwarf2_cu*, unsigned int, int) (read.c:23379)
    ==2140965==    by 0x552AB5: read_file_scope(die_info*, dwarf2_cu*) (read.c:9687)
    ==2140965==    by 0x54F7B2: process_die(die_info*, dwarf2_cu*) (read.c:8660)
    ==2140965==    by 0x5569C7: process_full_comp_unit (read.c:8429)
    ==2140965==    by 0x5569C7: process_queue (read.c:7675)
    ==2140965==    by 0x5569C7: dw2_do_instantiate_symtab (read.c:2063)
    ==2140965==    by 0x5569C7: dw2_instantiate_symtab(dwarf2_per_cu_data*, dwarf2_per_objfile*, bool) (read.c:2085)
    ==2140965==    by 0x55700B: dw2_expand_symtabs_matching_one(dwarf2_per_cu_data*, dwarf2_per_objfile*, gdb::function_view<bool (char const*, bool)>, gdb::function_view<bool (compunit_symtab*)>) (read.c:3984)
    ==2140965==    by 0x557EA3: cooked_index_functions::expand_symtabs_matching(objfile*, gdb::function_view<bool (char const*, bool)>, lookup_name_info const*, gdb::function_view<bool (char const*)>, gdb::function_view<bool (compunit_symtab*)>, enum_flags<block_search_flag_values>, domain_enum, search_domain) (read.c:18781)
    ==2140965==    by 0x778977: objfile::lookup_symbol(block_enum, char const*, domain_enum) (symfile-debug.c:276)
    ....
    ==2140965==  Uninitialised value was created by a heap allocation
    ==2140965==    at 0x4839F01: operator new(unsigned long) (vg_replace_malloc.c:434)
    ==2140965==    by 0x533A64: cutu_reader::cutu_reader(dwarf2_per_cu_data*, dwarf2_per_objfile*, abbrev_table*, dwarf2_cu*, bool, abbrev_cache*) (read.c:6264)
    ==2140965==    by 0x5340C2: load_full_comp_unit(dwarf2_per_cu_data*, dwarf2_per_objfile*, dwarf2_cu*, bool, language) (read.c:7729)
    ==2140965==    by 0x548338: load_cu(dwarf2_per_cu_data*, dwarf2_per_objfile*, bool) (read.c:2021)
    ==2140965==    by 0x55634C: dw2_do_instantiate_symtab (read.c:2048)
    ==2140965==    by 0x55634C: dw2_instantiate_symtab(dwarf2_per_cu_data*, dwarf2_per_objfile*, bool) (read.c:2085)
    ==2140965==    by 0x55700B: dw2_expand_symtabs_matching_one(dwarf2_per_cu_data*, dwarf2_per_objfile*, gdb::function_view<bool (char const*, bool)>, gdb::function_view<bool (compunit_symtab*)>) (read.c:3984)
    ==2140965==    by 0x557EA3: cooked_index_functions::expand_symtabs_matching(objfile*, gdb::function_view<bool (char const*, bool)>, lookup_name_info const*, gdb::function_view<bool (char const*)>, gdb::function_view<bool (compunit_symtab*)>, enum_flags<block_search_flag_values>, domain_enum, search_domain) (read.c:18781)
    ==2140965==    by 0x778977: objfile::lookup_symbol(block_enum, char const*, domain_enum) (symfile-debug.c:276)
    ....