[clangd] Desugar template parameter aliases in type hints

This patch alleviates https://github.com/clangd/clangd/issues/1298.

Containers in C++ such as `std::vector` or `llvm::SmallVector`,
introduce a series of type aliases to adapt to generic algorithms.

Currently, If we write an declarator involving expressions with
these containers and `auto` placeholder, we probably obtain opaque
type alias like following:

```
std::vector<int> v = {1, 2, 3};
auto value = v[1]; // hint for `value`: value_type
auto *ptr = &v[0]; // hint for `ptr`: value_type *
```

These hints are useless for most of the time. It would be nice if we
desugar the type of `value_type` and print `int`, `int *` respectively
in this situation. But note we can't always prefer desugared type
since user might introduce type-aliases for brevity, where printing
sugared types makes more sense.

This patch introduces a heuristic method that displays the desugared
type that is an alias of template parameter. It merges
analogous method `shouldPrintCanonicalType` into `maybeDesugar` as well.

Previous commit for shouldPrintCanonicalType: dde8a0fe

Reviewed By: nridge

Differential Revision: https://reviews.llvm.org/D151785