enum_flags: Fix ternary operator and remove implicit convertion to raw enum

The unit tests added by the previous patch revealed that this useful
use case doesn't work:

    enum flag { FLAG1 = 1, FLAG2 = 2 };
    enum_flags<flag> src = FLAG1;
    enum_flags<flag> f1 = condition ? src : FLAG2;

It fails to compile because enum_flags<flag> and flag are convertible
to each other.

Turns out that making enum_flags be implicitly convertible to the
backing raw enum type was not a good idea.

If we make it convertible to the underlying type instead, we fix that
ternary operator use case, and, we find cases throughout the codebase
that should be using the enum_flags but were using the raw backing
enum instead.  So it's a good change overall.

There's one case in compile/compile-c-types.c where we need to call a
function in a C API that expects the raw enum.  To address cases like
that, this adds a "raw()" method to enum_flags.  This way we can keep
using the safer enum_flags to construct the value, and then be
explicit when we need to get at the raw enum.

Tested with gcc 4.8, 4.9, 5.3, 7 (trunk) and clang 3.7.

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <palves@redhat.com>

	* common/enum-flags.h (enum_flags::operator&=): Change parameter
	type to enum_flags from enum_type and adjust.
	(enum_flags::operator|=): Likewise.
	(enum_flags::operator^=): Likewise.
	(enum_flags::operator enum_type): Delete.
	(enum_flags::raw): New method.
	(ENUM_FLAGS_GEN_BINOP): Adjust operator functions.
	* compile/compile-c-types.c (convert_qualified): Use
	enum_flags::raw.
	* enum-flags-selftests.c Adjust ternary operator CHECK_VALID
	tests.
	(selftests::enum_flags_tests::self_test): Add more ternary
	operator tests.
	* record-btrace.c (btrace_thread_flag_to_str): Change parameter's
	type to btrace_thread_flags from btrace_thread_flag.
	(record_btrace_cancel_resume, record_btrace_step_thread): Change
	local's type to btrace_thread_flags from btrace_thread_flag.  Add
	cast in DEBUG call.