Add minimal load-time DLL support on Windows, support dllimport storage class

src/compiler/crystal/codegen/codegen.cr

@@ -1450,6 +1450,9 @@ module Crystal
       unless var
         var = llvm_mod.globals.add(llvm_c_return_type(type), name)
         var.linkage = LLVM::Linkage::External
+        if @program.has_flag?("win32") && @program.has_flag?("preview_dll")
+          var.dll_storage_class = LLVM::DLLStorageClass::DLLImport
+        end
         var.thread_local = thread_local
       end
       var

src/empty.cr

@@ -1,7 +1,7 @@
 require "primitives"
 
 {% if flag?(:win32) %}
-  @[Link("libcmt")] # For `mainCRTStartup`
+  @[Link({{ flag?(:preview_dll) ? "msvcrt" : "libcmt" }})] # For `mainCRTStartup`
 {% end %}
 lib LibCrystalMain
   @[Raises]

src/lib_c.cr

@@ -1,5 +1,5 @@
 {% if flag?(:win32) %}
-  @[Link("libcmt")]
+  @[Link({{ flag?(:preview_dll) ? "msvcrt" : "libcmt" }})]
 {% end %}
 lib LibC
   alias Char = UInt8

src/llvm/enums.cr

@@ -220,15 +220,25 @@ module LLVM
     Appending
     Internal
     Private
-    DLLImport
-    DLLExport
+    DLLImport # obsolete
+    DLLExport # obsolete
     ExternalWeak
     Ghost
     Common
     LinkerPrivate
     LinkerPrivateWeak
   end
 
+  enum DLLStorageClass
+    Default
+
+    # Function to be imported from DLL.
+    DLLImport
+
+    # Function to be accessible from DLL.
+    DLLExport
+  end
+
   enum IntPredicate
     EQ  = 32
     NE

src/llvm/lib_llvm.cr

@@ -255,6 +255,7 @@ lib LibLLVM
   fun get_initializer = LLVMGetInitializer(global_var : ValueRef) : ValueRef
   fun set_linkage = LLVMSetLinkage(global : ValueRef, linkage : LLVM::Linkage)
   fun get_linkage = LLVMGetLinkage(global : ValueRef) : LLVM::Linkage
+  fun set_dll_storage_class = LLVMSetDLLStorageClass(global : ValueRef, storage_class : LLVM::DLLStorageClass)
   fun set_metadata = LLVMSetMetadata(value : ValueRef, kind_id : UInt32, node : ValueRef)
   fun set_target = LLVMSetTarget(mod : ModuleRef, triple : UInt8*)
   fun set_thread_local = LLVMSetThreadLocal(global_var : ValueRef, is_thread_local : Int32)

src/llvm/value_methods.cr

@@ -55,6 +55,10 @@ module LLVM::ValueMethods
     LibLLVM.get_linkage(self)
   end
 
+  def dll_storage_class=(storage_class)
+    LibLLVM.set_dll_storage_class(self, storage_class)
+  end
+
   def call_convention=(call_convention)
     LibLLVM.set_instruction_call_convention(self, call_convention)
   end