[clangd] Tune down quality score for class constructors so that it's ranked after class types.

Summary:
Currently, class constructors have the same score as the class types, and they
are often ranked before class types. This is often not desireable and can
be annoying when snippet is enabled and constructor signatures are added.

Metrics:

```
==================================================================================================
                                        OVERALL
==================================================================================================
  Total measurements: 111117 (+0)
  All measurements:
	MRR: 64.06 (+0.20)	Top-5: 75.73% (+0.14%)	Top-100: 93.71% (+0.01%)
  Full identifiers:
	MRR: 98.25 (+0.55)	Top-5: 99.04% (+0.03%)	Top-100: 99.16% (+0.00%)
  Filter length 0-5:
	MRR:      15.23 (+0.02)		50.50 (-0.02)		65.04 (+0.11)		70.75 (+0.19)		74.37 (+0.25)		79.43 (+0.32)
	Top-5:    40.90% (+0.03%)		74.52% (+0.03%)		87.23% (+0.15%)		91.68% (+0.08%)		93.68% (+0.14%)		95.87% (+0.12%)
	Top-100:  68.21% (+0.02%)		96.28% (+0.07%)		98.43% (+0.00%)		98.72% (+0.00%)		98.74% (+0.01%)		98.81% (+0.00%)
==================================================================================================
                                        DEFAULT
==================================================================================================
  Total measurements: 57535 (+0)
  All measurements:
	MRR: 58.07 (+0.37)	Top-5: 69.94% (+0.26%)	Top-100: 90.14% (+0.03%)
  Full identifiers:
	MRR: 97.13 (+1.05)	Top-5: 98.14% (+0.06%)	Top-100: 98.34% (+0.00%)
  Filter length 0-5:
	MRR:      13.91 (+0.00)		38.53 (+0.01)		55.58 (+0.21)		63.63 (+0.30)		69.23 (+0.47)		72.87 (+0.60)
	Top-5:    24.99% (+0.00%)		62.70% (+0.06%)		82.80% (+0.30%)		88.66% (+0.16%)		92.02% (+0.27%)		93.53% (+0.21%)
	Top-100:  51.56% (+0.05%)		93.19% (+0.13%)		97.30% (+0.00%)		97.81% (+0.00%)		97.85% (+0.01%)		97.79% (+0.00%)
```

Remark:
- The full-id completions have +1.05 MRR improvement.
- There is no noticeable impact on EXPLICIT_MEMBER_ACCESS and WANT_LOCAL.

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D49667

llvm-svn: 337816