[cxx-interop] Check the safety of C++ template arguments

Swift imports template specializations as a standalone type (not as an
instantiation of a generic) so unsafety is not propagated from the
template arguments to the specialization. This PR propagates this
information explicitly.

Author: Gabor Horvath

test/Interop/Cxx/class/safe-interop-mode.swift

@@ -88,14 +88,14 @@ func useCfType(x: CFArray) {
 func useString(x: std.string) {
 }
 
-// expected-warning@+1{{global function 'useVecOfPtr' involves unsafe code; use '@unsafe' to indicate that its use is not memory-safe}}
+// expected-warning@+1{{global function 'useVecOfPtr' has an interface that is not memory-safe; use '@unsafe' to indicate that its use is unsafe}}
 func useVecOfPtr(x: VecOfPtr) { // expected-note{{reference to unsafe type alias 'VecOfPtr'}}
 }
 
 func useVecOfInt(x: VecOfInt) {
 }
 
-// expected-warning@+1{{global function 'useCppSpan' involves unsafe code; use '@unsafe' to indicate that its use is not memory-safe}}
+// expected-warning@+1{{global function 'useCppSpan' has an interface that is not memory-safe; use '@unsafe' to indicate that its use is unsafe}}
 func useCppSpan(x: SpanOfInt) { // expected-note{{reference to unsafe type alias 'SpanOfInt'}}
 }