[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

include/swift/AST/AttrKind.h

@@ -105,6 +105,7 @@ enum : unsigned { NumEffectsKindBits =
 /// This enum represents the possible values of the @_expose attribute.
 enum class ExposureKind: uint8_t {
   Cxx,
+  NotCxx,
   Wasm,
   Last_ExposureKind = Wasm
 };

include/swift/AST/DiagnosticsSema.def

@@ -2132,6 +2132,8 @@ ERROR(expose_only_non_other_attr,none,
 ERROR(expose_inside_unexposed_decl,none,
       "'@_expose' cannot be applied inside of unexposed declaration %0",
       (const ValueDecl *))
+ERROR(expose_redundant_name_provided, none,
+      "'@_expose(!Cxx)' does not accept a name argument", ())
 ERROR(expose_invalid_name_pattern_init,none,
       "invalid declaration name '%0' specified in '@_expose'; "
       "exposed initializer name must start with 'init'", (StringRef))