[clang] [clang][Index][USR][NFC] Allow customizing langopts for USR generation (PR #109574)

[Balazs Benics via cfe-commits]

steakhal wrote:

For example the following code would get different USRs in C and C++:
```c++
static int f1(int* x);
// USR in C:   c:input.cc at F@f1
// USR in C++: c:input.cc at F@f1#*I#
```

So by having this patch, it would be possible to generate the C++ USR even if the AST (thus the ASTContext) was produced for a C compiler invocation.

https://github.com/llvm/llvm-project/pull/109574