[cxx-interop] Use clang::CanQualType in ClangTypeExplicitSafetyDescriptor

This will hopefully improve the cache hit rate.

Author: j-hui

include/swift/ClangImporter/ClangImporterRequests.h

@@ -625,9 +625,11 @@ void simple_display(llvm::raw_ostream &out, CxxValueSemanticsDescriptor desc);
 SourceLoc extractNearestSourceLoc(CxxValueSemanticsDescriptor desc);
 
 struct ClangTypeExplicitSafetyDescriptor final {
-  clang::QualType type;
+  clang::CanQualType type;
 
-  ClangTypeExplicitSafetyDescriptor(clang::QualType type) : type(type) {}
+  ClangTypeExplicitSafetyDescriptor(clang::CanQualType type) : type(type) {}
+  ClangTypeExplicitSafetyDescriptor(clang::QualType type)
+      : type(type->getCanonicalTypeUnqualified()) {}
 
   friend llvm::hash_code
   hash_value(const ClangTypeExplicitSafetyDescriptor &desc) {

lib/ClangImporter/ClangImporter.cpp

@@ -8642,8 +8642,8 @@ SourceLoc swift::extractNearestSourceLoc(SafeUseOfCxxDeclDescriptor desc) {
 
 void swift::simple_display(llvm::raw_ostream &out,
                            ClangTypeExplicitSafetyDescriptor desc) {
-  out << "Checking if type '" << desc.type.getAsString()
-      << "' is explicitly safe.\n";
+  auto qt = static_cast<clang::QualType>(desc.type);
+  out << "Checking if type '" << qt.getAsString() << "' is explicitly safe.\n";
 }
 
 SourceLoc swift::extractNearestSourceLoc(ClangTypeExplicitSafetyDescriptor desc) {
@@ -8652,7 +8652,7 @@ SourceLoc swift::extractNearestSourceLoc(ClangTypeExplicitSafetyDescriptor desc)
 
 ExplicitSafety ClangTypeExplicitSafety::evaluate(
     Evaluator &evaluator, ClangTypeExplicitSafetyDescriptor desc) const {
-  auto clangType = desc.type;
+  auto clangType = static_cast<clang::QualType>(desc.type);
 
   // Handle pointers.
   auto pointeeType = clangType->getPointeeType();