[gdb/build] Fix Wmaybe-uninitialized in gdb/ui-style.h

[gdb/build] Fix Wmaybe-uninitialized in gdb/ui-style.h

When building CFLAGS/CXXFLAGS="-O2 -g -Wall" and gcc 4.8.5, we run into:
...
src/gdb/cli/cli-style.c:154:42: warning: '*((void*)&<anonymous> +8)' \
  may be used uninitialized in this function [-Wmaybe-uninitialized]
src/gdb/cli/cli-style.c:154:42: warning: '*((void*)&<anonymous> +9)' \
  may be used uninitialized in this function [-Wmaybe-uninitialized]
src/gdb/cli/cli-style.c:154:42: warning: '*((void*)&<anonymous> +10)' \
  may be used uninitialized in this function [-Wmaybe-uninitialized]
...

The root cause is that the data members of class color, nested in struct
ui_file_style in gdb/ui-style.h:
...
    bool m_simple;
    int m_value;
    uint8_t m_red, m_green, m_blue;
...
are only partially initialized by this constructor:
...
    color (int c)
      : m_simple (true),
        m_value (c)
    {
      gdb_assert (c >= -1 && c <= 255);
    }
...
but the default copy constructor will copy all the fields.

The member m_simple acts as a discriminant, to indicate which other members
are valid:
- m_value (with m_simple == true)
- m_red, m_green, m_blue (with m_simple == false)
So, we don't need storage for both m_value and m_red/m_green/m_blue at the
same time.

Fix this by wrapping the respective members in a union:
...
    bool m_simple;
    union
    {
      int m_value;
      struct
      {
       uint8_t m_red, m_green, m_blue;
      };
    };
...
which also fixes the warning.

Build and reg-tested on x86_64-linux.

gdb/ChangeLog:

2020-07-30  Tom de Vries  <tdevries@suse.de>

	PR build/26320
	* ui-style.h (struct ui_file_style::color): Wrap m_value and
	m_red/m_green/m_blue in a union.