CMake: Make most target symbols hidden by default

Summary:
For builds with LLVM_BUILD_LLVM_DYLIB=ON and BUILD_SHARED_LIBS=OFF
this change makes all symbols in the target specific libraries hidden
by default.

A new macro called LLVM_EXTERNAL_VISIBILITY has been added to mark symbols in these
libraries public, which is mainly needed for the definitions of the
LLVMInitialize* functions.

This patch reduces the number of public symbols in libLLVM.so by about
25%.  This should improve load times for the dynamic library and also
make abi checker tools, like abidiff require less memory when analyzing
libLLVM.so

One side-effect of this change is that for builds with
LLVM_BUILD_LLVM_DYLIB=ON and LLVM_LINK_LLVM_DYLIB=ON some unittests that
access symbols that are no longer public will need to be statically linked.

Before and after public symbol counts (using gcc 8.2.1, ld.bfd 2.31.1):
nm before/libLLVM-9svn.so | grep ' [A-Zuvw] ' | wc -l
36221
nm after/libLLVM-9svn.so | grep ' [A-Zuvw] ' | wc -l
26278

Reviewers: chandlerc, beanz, mgorny, rnk, hans

Reviewed By: rnk, hans

Subscribers: merge_guards_bot, luismarques, smeenai, ldionne, lenary, s.egerton, pzheng, sameer.abuasal, MaskRay, wuzish, echristo, Jim, hiraditya, michaelplatings, chapuni, jholewinski, arsenm, dschuff, jyknight, dylanmckay, sdardis, nemanjai, jvesely, javed.absar, sbc100, jgravelle-google, aheejin, kbarton, fedor.sergeev, asb, rbar, johnrusso, simoncook, apazos, sabuasal, niosHD, jrtc27, zzheng, edward-jones, mgrang, atanasyan, rogfer01, MartinMosbeck, brucehoult, the_o, PkmX, jocewei, kristina, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D54439

1 files changed, 1 insertions, 1 deletions

diff --git a/llvm/lib/Target/WebAssembly/TargetInfo/WebAssemblyTargetInfo.cpp b/llvm/lib/Target/WebAssembly/TargetInfo/WebAssemblyTargetInfo.cpp
index e4afe2bb283..87317f8a7f1 100644
--- a/llvm/lib/Target/WebAssembly/TargetInfo/WebAssemblyTargetInfo.cpp
+++ b/llvm/lib/Target/WebAssembly/TargetInfo/WebAssemblyTargetInfo.cpp
@@ -26,7 +26,7 @@ Target &llvm::getTheWebAssemblyTarget64() {
   return TheWebAssemblyTarget64;
 }
 
-extern "C" void LLVMInitializeWebAssemblyTargetInfo() {
+extern "C" LLVM_EXTERNAL_VISIBILITY void LLVMInitializeWebAssemblyTargetInfo() {
   RegisterTarget<Triple::wasm32> X(getTheWebAssemblyTarget32(), "wasm32",
                                    "WebAssembly 32-bit", "WebAssembly");
   RegisterTarget<Triple::wasm64> Y(getTheWebAssemblyTarget64(), "wasm64",