Turn on WITH_LLVM_CXXFILT by default

LLVM's demangler supports more modern C++ constructs such as lambdas and
unnamed types, and is actively maintained. The command line tool is
usable as a drop-in replacement for GNU c++filt, or elftoolchain's
cxxfilt. The latter is still available by using WITHOUT_LLVM_CXXFILT, if
needed.

PR:		250702
MFC after:	2 weeks

share/man/man5/src.conf.5

@@ -1,6 +1,6 @@
 .\" DO NOT EDIT-- this file is @generated by tools/build/options/makeman.
 .\" $FreeBSD$
-.Dd November 3, 2020
+.Dd November 4, 2020
 .Dt SRC.CONF 5
 .Os
 .Sh NAME
@@ -919,8 +919,8 @@ Set to disable debugging assertions in LLVM.
 Set to not build the
 .Xr llvm-cov 1
 tool.
-.It Va WITH_LLVM_CXXFILT
+.It Va WITHOUT_LLVM_CXXFILT
-Install LLVM's llvm-cxxfilt as c++filt, instead of ELF Tool Chain's cxxfilt.
+Install ELF Tool Chain's cxxfilt as c++filt, instead of LLVM's llvm-cxxfilt.
 .It Va WITHOUT_LLVM_TARGET_AARCH64
 Set to not build LLVM target support for AArch64.
 The

share/mk/src.opts.mk

@@ -135,6 +135,7 @@ __DEFAULT_YES_OPTIONS = \
     LLD_IS_LD \
     LLVM_ASSERTIONS \
     LLVM_COV \
+    LLVM_CXXFILT \
     LLVM_TARGET_ALL \
     LOADER_GELI \
     LOADER_LUA \
@@ -210,7 +211,6 @@ __DEFAULT_NO_OPTIONS = \
     GNU_GREP_COMPAT \
     HESIOD \
     LIBSOFT \
-    LLVM_CXXFILT \
     LOADER_FIREWIRE \
     LOADER_VERBOSE \
     LOADER_VERIEXEC_PASS_MANIFEST \