src.opts.mk: Default LLVM_ASSERTIONS off

This noticeably slows down the compiler execution and with the move to
pkgbase is one of the remaining differences that cannot be
administratively controlled; pkg kernels and malloc.conf(5) will allow
quickly testing a snapshot in a configuration closer to stable and
release branches.

dim@ notes that you need to set WITH_LLVM_ASSERTIONS in src.conf(5) if
you want to report an issue with LLVM.

Enabling WITH_LLVM_ASSERTIONS is also a consideration for CI and pkg
builders especially tracking main.

Approved by:	dim, kevans
Differential Revision:	https://reviews.freebsd.org/D50388

share/mk/src.opts.mk

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