[cxx-interop] Add attribute to hide Swift declarations from interop

Explanation: We generate declarations in the C++ interop header with
"unavailable" annotations when we cannot export something to C++. These
declarations can collide with existing names. Previously, there were no
ways to resolve these name collisions. This PR introduces a new
attribute to hide declarations from the interop header.
Issues: rdar://158843666
Original PRs: #82616
Risk: Low, this adds a new, straightforward code path.
Testing: Added a compiler test.
Reviewers: @egorzhdan

lib/PrintAsClang/ModuleContentsWriter.cpp

@@ -1018,7 +1018,8 @@ public:
     // library. Also skip structs from the standard library, they can cause
     // ambiguities because of the arithmetic types that conflict with types we
     // already have in `swift::` namespace. Also skip `Error` protocol from
-    // stdlib, we have experimental support for it.
+    // stdlib, we have experimental support for it. Also skip explicitly exluded
+    // declarations.
     removedVDList.erase(
         llvm::remove_if(
             removedVDList,
@@ -1028,7 +1029,8 @@ public:
                       vd->getBaseIdentifier().hasUnderscoredNaming()) ||
                      (vd->isStdlibDecl() && isa<StructDecl>(vd)) ||
                      (vd->isStdlibDecl() &&
-                      vd->getASTContext().getErrorDecl() == vd);
+                      vd->getASTContext().getErrorDecl() == vd) ||
+                     swift::hasExposeNotCxxAttr(vd);
             }),
         removedVDList.end());
     // Sort the unavaiable decls by their name and kind.