ImportC: Support weird asm("_" "<name>") mangling stuff (#4486)

These are apparently used in the Mac system headers...

Co-authored-by: Martin Kinkelin <kinke@gnx.net>

Author: kinke

gen/mangling.cpp

@@ -98,6 +98,9 @@ std::string hashSymbolName(llvm::StringRef name, Dsymbol *symb) {
 
 std::string getIRMangledName(FuncDeclaration *fdecl, LINK link) {
   std::string mangledName = mangleExact(fdecl);
+  if (fdecl->adFlags & 4) { // nounderscore
+    mangledName.insert(0, "\1");
+  }
 
   // Hash the name if necessary
   if (((link == LINK::d) || (link == LINK::default_)) &&

tests/dmd/runnable/test23343.c

@@ -1,6 +1,4 @@
-/* DISABLED: win32 win64 linux32 linux64 freebsd32 freebsd64 osx32 dragonflybsd32 netbsd32 LDC
- * LDC: this was apparently hacked around in DMD, requiring the glue layer to
-        know about this ImportC special case and only apply it for Mac targets...
+/* DISABLED: win32 win64 linux32 linux64 freebsd32 freebsd64 osx32 dragonflybsd32 netbsd32
  */
 
 /* https://issues.dlang.org/show_bug.cgi?id=23343