[Support] Use HAVE_DLOPEN to guard dlopen(3) usage

[labath]

Summary

The usage was previously guarded by HAVE_DLFCN. This breaks on Android with
LLVM_BUILD_STATIC as the platform does not provide a static version of libdl.
Using HAVE_DLOPEN fixes it as the code will only get used if we are actually able
to link an executable using dlopen.

[labath]

Do you have any objections to this going in? (If I chose the wrong reviewers, let me know).

[joerg]

Please use #if defined(HAVE_DLFCN_H) && defined(HAVE_DLOPEN) in both places, otherwise LGTM.

[labath]

Thank you for checking this out.

HAVE_DLOPEN is defined only if HAVE_DLFCN_H is detected, but I suppose it doesn't hurt to be explicit.

lib/Support/DynamicLibrary.cpp

 char llvm::sys::DynamicLibrary::Invalid = 0;
 
 #ifdef LLVM_ON_WIN32
 
 #include "Windows/DynamicLibrary.inc"
 
 #else
 
-#if HAVE_DLFCN_H
+#if defined(HAVE_DLFCN_H) && defined(HAVE_DLOPEN)
 #include <dlfcn.h>
 using namespace llvm;
 using namespace llvm::sys;
 
 //===----------------------------------------------------------------------===//
 //=== WARNING: Implementation here must contain only TRULY operating system
 //===          independent code.
 //===----------------------------------------------------------------------===//
void* DynamicLibrary::SearchForAddressOfSymbol(const char *symbolName) {
   // First check symbols added via AddSymbol().
   if (ExplicitSymbols.isConstructed()) {
     StringMap<void *>::iterator i = ExplicitSymbols->find(symbolName);
 
     if (i != ExplicitSymbols->end())
       return i->second;
   }
 
-#if HAVE_DLFCN_H
+#if defined(HAVE_DLFCN_H) && defined(HAVE_DLOPEN)
   // Now search the libraries.
   if (OpenedHandles) {
     for (DenseSet<void *>::iterator I = OpenedHandles->begin(),
          E = OpenedHandles->end(); I != E; ++I) {
       //lt_ptr ptr = lt_dlsym(*I, symbolName);
       void *ptr = dlsym(*I, symbolName);
       if (ptr) {
         return ptr;