[gdb] Rewrite error and warning emojis

While working on PR34501 I realized that these two strings actually contain
two unicode characters:
...
static std::string warning_prefix = "⚠️ ";
static std::string error_prefix = "❌️ ";
...

Both the Warning Sign [1] and the Cross Mark [2] are followed by Variation
Selector-16 (VS16) [3].

To make this obvious, I decided to rewrite in a style that makes it explicit
both which unicode characters are used, and how they are encoded:
...
static std::string warning_prefix = u8"\u26A0\uFE0F ";
...

In the process I found out that the Cross Mark doesn't need VS16, because its
default presentation is already "Emoji" rather than "Text", so that one simply
becomes:
...
static std::string error_prefix = u8"\u274C ";
...

AFAICT, this property (default presentation == "Emoji") can be verified by
finding Cross Mark here [4] and checking that property Emoji_Presentation
applies.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

[1] https://www.compart.com/en/unicode/U+26A0
[2] https://www.compart.com/en/unicode/U+274C
[3] https://www.compart.com/en/unicode/U+FE0F
[4] https://www.unicode.org/Public/17.0.0/ucd/emoji/emoji-data.txt

gdb/cli/cli-style.c

diff --git a/gdb/cli/cli-style.c b/gdb/cli/cli-style.c
index 03a8872..5c175ee 100644
--- a/gdb/cli/cli-style.c
+++ b/gdb/cli/cli-style.c
@@ -449,8 +449,12 @@ no_emojis ()
   emoji_styling = AUTO_BOOLEAN_FALSE;
 }
 
-/* Emoji warning prefix.  */
-static std::string warning_prefix = "⚠️ ";
+/* Emoji warning prefix:
+   - u26A0: Warning Sign: ⚠
+   - uFE0F: Variation Selector-16 (VS16)
+   The VS16 forces "Emoji" presentation.  It is needed because the default
+   presentation for Warning Sign is "Text".  Together, we get: ⚠️ .  */
+static std::string warning_prefix = u8"\u26A0\uFE0F ";
 
 /* Implement 'show style warning-prefix'.  */
 
@@ -471,8 +475,11 @@ print_warning_prefix (ui_file *file)
     gdb_puts (warning_prefix.c_str (), file);
 }
 
-/* Emoji error prefix.  */
-static std::string error_prefix = "❌️ ";
+/* Emoji error prefix:
+   - u274C: Cross Mark: ❌
+   No VS16 is needed because the default presentation for Cross Mark is
+   "Emoji".  */
+static std::string error_prefix = u8"\u274C ";
 
 /* Implement 'show style error-prefix'.  */
 

gdb/testsuite/gdb.base/style.exp

diff --git a/gdb/testsuite/gdb.base/style.exp b/gdb/testsuite/gdb.base/style.exp
index 8a49d1c..8d576ff 100644
--- a/gdb/testsuite/gdb.base/style.exp
+++ b/gdb/testsuite/gdb.base/style.exp
@@ -335,7 +335,7 @@
 	gdb_test_no_output "set host-charset UTF-8"
 	# Chosen since it will print an error.
 	gdb_test "maint translate-address" \
-	    "❌️ requires argument.*" \
+	    "❌ requires argument.*" \
 	    "emoji output"
 
 	gdb_test_no_output "set style error-prefix abcd:" \