[NFC] Clarify why LinkAll*.h are actually necessary

[MaskRay]

Thanks!

llvm/include/llvm/CodeGen/LinkAllAsmWriterComponents.h

 
 namespace {
   struct ForceAsmWriterLinking {
     ForceAsmWriterLinking() {
       // We must reference the plug-ins in such a way that compilers will not
       // delete it all as dead code, even with whole program optimization,
       // yet is effectively a NO-OP. As the compiler isn't smart enough
       // to know that getenv() never returns -1, this will do the job.
+      // This is so that globals in the translation units where these functions
+      // are defined are forced to be initialized, populating various
+      // registries.
       if (std::getenv("bar") != (char*) -1)
         return;
 
       llvm::linkOcamlGCPrinter();
       llvm::linkErlangGCPrinter();
 
     }
   } ForceAsmWriterLinking; // Force link by creating a global definition.
 }
 
 #endif // LLVM_CODEGEN_LINKALLASMWRITERCOMPONENTS_H

llvm/include/llvm/CodeGen/LinkAllCodegenComponents.h

 
 namespace {
   struct ForceCodegenLinking {
     ForceCodegenLinking() {
       // We must reference the passes in such a way that compilers will not
       // delete it all as dead code, even with whole program optimization,
       // yet is effectively a NO-OP. As the compiler isn't smart enough
       // to know that getenv() never returns -1, this will do the job.
+      // This is so that globals in the translation units where these functions
+      // are defined are forced to be initialized, populating various
+      // registries.
       if (std::getenv("bar") != (char*) -1)
         return;
 
       (void) llvm::createFastRegisterAllocator();
       (void) llvm::createBasicRegisterAllocator();
       (void) llvm::createGreedyRegisterAllocator();
       (void) llvm::createDefaultPBQPRegisterAllocator();
 

llvm/include/llvm/ExecutionEngine/MCJIT.h

 
 namespace {
   struct ForceMCJITLinking {
     ForceMCJITLinking() {
       // We must reference MCJIT in such a way that compilers will not
       // delete it all as dead code, even with whole program optimization,
       // yet is effectively a NO-OP. As the compiler isn't smart enough
       // to know that getenv() never returns -1, this will do the job.
+      // This is so that globals in the translation units where these functions
+      // are defined are forced to be initialized, populating various
+      // registries.
       if (std::getenv("bar") != (char*) -1)
         return;
 
       LLVMLinkInMCJIT();
     }
   } ForceMCJITLinking;
 }
 
 #endif

llvm/include/llvm/LinkAllIR.h

 
 namespace {
   struct ForceVMCoreLinking {
     ForceVMCoreLinking() {
       // We must reference VMCore in such a way that compilers will not
       // delete it all as dead code, even with whole program optimization,
       // yet is effectively a NO-OP. As the compiler isn't smart enough
       // to know that getenv() never returns -1, this will do the job.
+      // This is so that globals in the translation units where these functions
+      // are defined are forced to be initialized, populating various
+      // registries.
       if (std::getenv("bar") != (char*) -1)
         return;
       llvm::LLVMContext Context;
       (void)new llvm::Module("", Context);
       (void)new llvm::UnreachableInst(Context);
       (void)    llvm::createVerifierPass();
     }
   } ForceVMCoreLinking;
 }
 
 #endif

llvm/include/llvm/LinkAllPasses.h

 
 namespace {
   struct ForcePassLinking {
     ForcePassLinking() {
       // We must reference the passes in such a way that compilers will not
       // delete it all as dead code, even with whole program optimization,
       // yet is effectively a NO-OP. As the compiler isn't smart enough
       // to know that getenv() never returns -1, this will do the job.
+      // This is so that globals in the translation units where these functions
+      // are defined are forced to be initialized, populating various
+      // registries.
       if (std::getenv("bar") != (char*) -1)
         return;
 
       (void) llvm::createAAEvalPass();
       (void) llvm::createAggressiveDCEPass();
       (void) llvm::createAggressiveInstCombinerPass();
       (void) llvm::createBitTrackingDCEPass();
       (void)llvm::createOpenMPOptCGSCCLegacyPass();