[gdb/symtab] Fix version check in dwarf compilation unit header

[gdb/symtab] Fix version check in dwarf compilation unit header

The version check of the dwarf compilation unit header in
error_check_comp_unit_head is done too late, and consequently dwarf code with
an unsupported version in the compilation unit header is interpreted as dwarf5
code in read_comp_unit_head.

Fixed by moving the check earlier.

Build and reg-tested on x86_64-linux.

2018-07-04  Tom de Vries  <tdevries@suse.de>

	* dwarf2read.c (error_check_comp_unit_head): Move dwarf version
	check ...
	(read_comp_unit_head): ... here.

	* gdb.dwarf2/dw2-error.exp: Update expected error message.