[lldb] Support any flag to _regexp-bt (#116260)

In particular, this allows `bt -u`.

Note that this passthrough behavior has precedent in `_regexp-break`,
where `b (-.*)` is expanded to `breakpoint set %1`.

lldb/source/Interpreter/CommandInterpreter.cpp

diff --git a/lldb/source/Interpreter/CommandInterpreter.cpp b/lldb/source/Interpreter/CommandInterpreter.cpp
index f2712af..764dcfd 100644
--- a/lldb/source/Interpreter/CommandInterpreter.cpp
+++ b/lldb/source/Interpreter/CommandInterpreter.cpp
@@ -851,8 +851,7 @@
     // now "bt 3" is the preferred form, in line with gdb.
     if (bt_regex_cmd_up->AddRegexCommand("^([[:digit:]]+)[[:space:]]*$",
                                          "thread backtrace -c %1") &&
-        bt_regex_cmd_up->AddRegexCommand("^-c ([[:digit:]]+)[[:space:]]*$",
-                                         "thread backtrace -c %1") &&
+        bt_regex_cmd_up->AddRegexCommand("^(-[^[:space:]].*)$", "thread backtrace %1") &&
         bt_regex_cmd_up->AddRegexCommand("^all[[:space:]]*$", "thread backtrace all") &&
         bt_regex_cmd_up->AddRegexCommand("^[[:space:]]*$", "thread backtrace")) {
       CommandObjectSP command_sp(bt_regex_cmd_up.release());

lldb/test/API/lang/cpp/std-function-recognizer/TestStdFunctionRecognizer.py

diff --git a/lldb/test/API/lang/cpp/std-function-recognizer/TestStdFunctionRecognizer.py b/lldb/test/API/lang/cpp/std-function-recognizer/TestStdFunctionRecognizer.py
index d1cb821..978bf20 100644
--- a/lldb/test/API/lang/cpp/std-function-recognizer/TestStdFunctionRecognizer.py
+++ b/lldb/test/API/lang/cpp/std-function-recognizer/TestStdFunctionRecognizer.py
@@ -47,6 +47,11 @@
         )
         # Unfiltered.
         self.expect(
+            "bt -u",
+            ordered=True,
+            patterns=["frame.*foo", "frame.*std::__[^:]*::__function", "frame.*main"],
+        )
+        self.expect(
             "thread backtrace -u",
             ordered=True,
             patterns=["frame.*foo", "frame.*std::__[^:]*::__function", "frame.*main"],