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#`.
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