[MinGW] Don't currently set visibility=hidden when building for MinGW

[mstorsjo]

Summary

If we build the Target libraries with -fvisibility=hidden, then
LLVM_EXTERNAL_VISIBILITY must also be able to override it back
to default visibility.

Currently, the LLVM_EXTERNAL_VISIBILITY define is a no-op for
mingw targets, thus set CMAKE_CXX_VISIBILITY_PRESET correspondingly.

This unbreaks the mingw dylib build, if the compiler actually
takes hidden visiblity into account (e.g. after D130121).

(Later, once hidden visiblity can be used for MinGW targets, we
can make LLVM_EXTERNAL_VISIBILITY and LLVM_LIBRARY_VISIBILITY expand
to actual attributes, and reverse this commit.)

[rnk]

Tom added hidden Target symbol visibility originally, right, can he review? +@tstellar

This seems reasonable to me.

[mati865]

LGTM

llvm/lib/Target/CMakeLists.txt

add_llvm_component_library(LLVMTarget
   Core
   MC
   Support
   )
 
 # When building shared objects for each target there are some internal APIs
 # that are used across shared objects which we can't hide.
 if (NOT BUILD_SHARED_LIBS AND NOT APPLE AND
+    NOT MINGW AND
     NOT (${CMAKE_SYSTEM_NAME} MATCHES "AIX") AND
     NOT DEFINED CMAKE_CXX_VISIBILITY_PRESET)
   # Set default visibility to hidden, so we don't export all the Target classes
   # in libLLVM.so.
   set(CMAKE_CXX_VISIBILITY_PRESET hidden)
 endif()
 
 foreach(t ${LLVM_TARGETS_TO_BUILD})