[clang][Index] Use canonical function parameter types in USRs

[sdkrystian]

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 the proposed changes, the USR for both functions will be c:@F@f#I#.

[mizvekov]

Also, would you make sure to add a test case for this?

Thanks.

[sdkrystian]

@mizvekov Sure, I'll take care of that tomorrow

[mizvekov]

@sdkrystian ping, do you still intend to continue this? Just adding your example as a test case would be fine.

[sdkrystian]

@mizvekov I totally forgot about this PR, sorry! I'll add a test and merge tomorrow :)