Fix build failure on macOS

Fix build failure on macOS

PR build/25250 notes that the gdb 9 pre-release fails to build on
macOS, due to a name clash between field_kind::STRING and the STRING
token in ada-exp.y.  I am not sure (I couldn't reproduce this myself),
but presumably this is due to differences caused by the version of
bison in use there.

This patch works around the problem by renaming the field_kind
enumerator.  I chose to rename this one because it is used in
relatively few places -- it's just an implementation detail of the
style code.

This version also renames field_kind::SIGNED for consistency.

Let me know what you think.  I intend to check this in on the gdb 9
branch as well.

gdb/ChangeLog
2019-12-18  Tom Tromey  <tromey@adacore.com>

	PR build/25250:
	* ui-out.c (ui_out::vmessage): Update.
	* ui-out.h (enum class field_kind) <FIELD_STRING, FIELD_SIGNED>:
	Rename.
	(string_field): Update.
	(signed_field): Update.

Change-Id: Iae9f36f1b793e22c61fee0de2ab2d508668ee7e4