[clang][Index] Use canonical function parameter types in USRs (#68222)

This is necessary to ensure that functions declared in different
translation units whose parameter types only differ in top-level
cv-qualification generate the same USR.

For example:
```
// A.cpp
void f(const int x); // c:@f@f#1I#

// B.cpp
void f(int x);       // c:@f@f#I#
``` 
With this patch, the USR for both functions will be
`c:@f@f#I#`.

clang/lib/Index/USRGeneration.cpp

@@ -267,10 +267,13 @@ void USRGenerator::VisitFunctionDecl(const FunctionDecl *D) {
     Out << '>';
   }
 
+  QualType CanonicalType = D->getType().getCanonicalType();
   // Mangle in type information for the arguments.
-  for (auto *PD : D->parameters()) {
-    Out << '#';
-    VisitType(PD->getType());
+  if (const auto *FPT = CanonicalType->getAs<FunctionProtoType>()) {
+    for (QualType PT : FPT->param_types()) {
+      Out << '#';
+      VisitType(PT);
+    }
   }
   if (D->isVariadic())
     Out << '.';

clang/test/Index/USR/func-type.cpp

@@ -16,3 +16,15 @@ void Func( void (* (*)(int, int))(int, int) );
 // CHECK: {{[0-9]+}}:6 | function/C | Func | c:@F@Func#*F*Fv(#I#I)(#I#I)# |
 void Func( void (* (*)(int, int, int))(int) );
 // CHECK: {{[0-9]+}}:6 | function/C | Func | c:@F@Func#*F*Fv(#I)(#I#I#I)# |
+
+// Functions with parameter types that only differ in top-level cv-qualification should generate the same USR.
+
+void f( const int );
+// CHECK: {{[0-9]+}}:6 | function/C | f | c:@F@f#I# |
+void f( int );
+// CHECK: {{[0-9]+}}:6 | function/C | f | c:@F@f#I# |
+
+void g( int );
+// CHECK: {{[0-9]+}}:6 | function/C | g | c:@F@g#I# |
+void g( const int );
+// CHECK: {{[0-9]+}}:6 | function/C | g | c:@F@g#I# |