Reduce number of external symbols in libllvm, libclang and liblldb

In commit 2e47f35be5 libllvm, libclang and liblldb were converted into
private shared libraries. This allowed clang, lld, lldb, and other llvm
tools to be linked against these shared libraries, which makes them
smaller and avoids duplication.

However, upstream builds the shared libraries using several visibility
options, which reduces the number of external symbols, and makes the
libraries a bit smaller.

On my test machine:
* libprivatellvm.so goes from 75643 to 34706 symbols (~54% reduction)
* libprivateclang.so goes from 53250 to 33531 symbols (~37% reduction)
* libprivatelldb.so goes from 27242 to 18798 symbols (~31% reduction)

Note: to get the full benefit, a clean build is required. Incremental
builds should still work, but I didn't want to force a full rebuild on
everybody.

MFC after:	1 week

lib/clang/libllvm/Makefile

@@ -48,6 +48,8 @@ CFLAGS+=	-DBLAKE3_NO_AVX512 -DBLAKE3_NO_AVX2 -DBLAKE3_NO_SSE41 \
 CFLAGS+=	-DBLAKE3_USE_NEON=0
 .endif
 
+CFLAGS+=	-DLLVM_BUILD_LLVM_DYLIB
+
 SRCDIR=		llvm/lib
 
 # Explanation of different SRCS variants below:
@@ -2050,6 +2052,16 @@ SRCS_ALL+=	${SRCS_COV}
 SRCS+=		${GENSRCS}
 SRCS+=		${SRCS_ALL:O}
 
+# Optimization: for all source files under Target/*/*, use -fvisibility=hidden,
+# which reduces the number of external symbols in the resulting shared library.
+# This is also what upstream does.
+.for src in ${SRCS}
+. if "${src:MTarget/*/*}"
+CFLAGS.${src:T}+=	-fvisibility=hidden
+.info ${src}:	CFLAGS.${src:T}
+. endif
+.endfor
+
 llvm/CodeGen/GenVT.inc: ${LLVM_SRCS}/include/llvm/CodeGen/ValueTypes.td
 	${LLVM_MIN_TBLGEN} -gen-vt \
 	    -I ${LLVM_SRCS}/include -d ${.TARGET}.d -o ${.TARGET} \

lib/clang/llvm.build.mk

@@ -122,6 +122,7 @@ LDFLAGS+=	-Wl,-m,elf64lriscv_fbsd
 CXXSTD=		c++17
 CXXFLAGS+=	-fno-exceptions
 CXXFLAGS+=	-fno-rtti
+CXXFLAGS+=	-fvisibility-inlines-hidden
 .if ${.MAKE.OS} == "FreeBSD" || !defined(BOOTSTRAPPING)
 CXXFLAGS.clang+= -stdlib=libc++
 .else