Revert "src.opts.mk: Default LLVM_ASSERTIONS off"

Enabling and disabling LLVM assertions changes the ABI. There have
been multiple reports on the freebsd-current@ mailing list of broken
'cc'.

Out of caution, you may wish to do a clean build while crossing this
commit after building the reverted commit if you do not set
LLVM_ASSERTIONS in your src.conf.

Discussed with:	kevans, jrtc27

This reverts commit 9b3055d0d4.

share/mk/src.opts.mk

@@ -123,6 +123,7 @@ __DEFAULT_YES_OPTIONS = \
     LEGACY_CONSOLE \
     LLD \
     LLD_BOOTSTRAP \
+    LLVM_ASSERTIONS \
     LLVM_BINUTILS \
     LLVM_COV \
     LLVM_CXXFILT \
@@ -209,7 +210,6 @@ __DEFAULT_NO_OPTIONS = \
     HESIOD \
     LOADER_VERBOSE \
     LOADER_VERIEXEC_PASS_MANIFEST \
-    LLVM_ASSERTIONS \
     LLVM_FULL_DEBUGINFO \
     MALLOC_PRODUCTION \
     OFED_EXTRA \