dwarf2read: Silence -Wenum-compare-switch warning

dwarf2read: Silence -Wenum-compare-switch warning

Compiling with Clang 6 gives:

/home/emaisin/src/binutils-gdb/gdb/dwarf2read.c:24385:14: error: comparison of two values with different enumeration types in switch statement ('enum dwarf_macro_record_type' and 'dwarf_macinfo_record_type') [-Werror,-Wenum-compare-switch]
        case DW_MACINFO_vendor_ext:
             ^~~~~~~~~~~~~~~~~~~~~
/home/emaisin/src/binutils-gdb/gdb/dwarf2read.c:24561:7: error: comparison of two values with different enumeration types in switch statement ('enum dwarf_macro_record_type' and 'dwarf_macinfo_record_type') [-Werror,-Wenum-compare-switch]
        case DW_MACINFO_vendor_ext:
             ^~~~~~~~~~~~~~~~~~~~~

This code uses the two enum types on purpose, because it handles both
.debug_macro and .debug_macinfo sections.  Add some pragmas to disable
the warning in these specific cases.

gdb/ChangeLog:

	* dwarf2read.c (dwarf_decode_macro_bytes): Ignore
	-Wenum-compare-switch warning.
	(dwarf_decode_macros): Likewise.