Add DW_OP_LLVM_extend DWARF operation

Previous changes allow a new set of more complex DWARF expression
operations to be added which are very usefull for SIMD/SIMT like
architectures. First of which is the DW_OP_LLVM_extend operation
that pops one stack element (which must be a location description)
and treat it as a number of pieces of a new composite location
description.

This means that a resulting composite location contains a given
number of pieces of a given bit size, where all the pieces are
described by the same location description found on top of the stack.

gdb/ChangeLog:

        * compile/compile-loc2c.c (compute_stack_depth_worker): Add
        new DW_OP_LLVM_extend operation support.
        * dwarf2/expr.c (dwarf_expr_context::create_extend_composite):
        New method that creates the extend composite.
        (dwarf_expr_context::execute_stack_op): Add new
        DW_OP_LLVM_extend operation support.
        * dwarf2/loc.c (dwarf2_get_symbol_read_needs): Add new
        DW_OP_LLVM_extend operation support.
        (disassemble_dwarf_expression): Add new DW_OP_LLVM_extend
        operation support.

include/ChangeLog:

        * dwarf2.def: Add new DW_OP_LLVM_extend enumeration.

gdb/testsuite/ChangeLog:

        * gdb.dwarf2/dw2-llvm-extend.exp: New test.
        * lib/dwarf.exp: Add new DW_OP_LLVM_extend operation support.
6 files changed