add module flag to emit CodeView for COFF object files

see #516

README.md

@@ -189,6 +189,10 @@ make install
 ./zig build --build-file ../build.zig test
 ```
 
+#### Windows
+
+See https://github.com/zig-lang/zig/wiki/Building-Zig-on-Windows
+
 ### Release / Install Build
 
 Once installed, `ZIG_LIBC_LIB_DIR` and `ZIG_LIBC_INCLUDE_DIR` can be overridden

src/codegen.cpp

@@ -4992,7 +4992,11 @@ static void init(CodeGen *g) {
 
     LLVMSetTarget(g->module, buf_ptr(&g->triple_str));
 
-    ZigLLVMAddModuleDebugInfoFlag(g->module);
+    if (g->zig_target.oformat == ZigLLVM_COFF) {
+        ZigLLVMAddModuleCodeViewFlag(g->module);
+    } else {
+        ZigLLVMAddModuleDebugInfoFlag(g->module);
+    }
 
     LLVMTargetRef target_ref;
     char *err_msg = nullptr;

src/zig_llvm.cpp

@@ -706,6 +706,10 @@ void ZigLLVMAddModuleDebugInfoFlag(LLVMModuleRef module) {
     unwrap(module)->addModuleFlag(Module::Warning, "Debug Info Version", DEBUG_METADATA_VERSION);
 }
 
+void ZigLLVMAddModuleCodeViewFlag(LLVMModuleRef module) {
+    unwrap(module)->addModuleFlag(Module::Warning, "CodeView", 1);
+}
+
 static AtomicOrdering mapFromLLVMOrdering(LLVMAtomicOrdering Ordering) {
     switch (Ordering) {
         case LLVMAtomicOrderingNotAtomic: return AtomicOrdering::NotAtomic;

src/zig_llvm.hpp

@@ -112,6 +112,7 @@ unsigned ZigLLVMTag_DW_structure_type(void);
 
 ZigLLVMDIBuilder *ZigLLVMCreateDIBuilder(LLVMModuleRef module, bool allow_unresolved);
 void ZigLLVMAddModuleDebugInfoFlag(LLVMModuleRef module);
+void ZigLLVMAddModuleCodeViewFlag(LLVMModuleRef module);
 
 void ZigLLVMSetCurrentDebugLocation(LLVMBuilderRef builder, int line, int column, ZigLLVMDIScope *scope);
 void ZigLLVMClearCurrentDebugLocation(LLVMBuilderRef builder);