Do not cache hardcoded formats in FormatManager

The cache in FormatCache uses only a type name as key. The hardcoded
formats, synthetic children, etc inspect an entire ValueObject to
determine their eligibility, which isn't modelled in the cache. This
leads to bugs such as the one in this patch (where two similarly named
types in different files have different hardcoded summary
providers). The problem is exaggerated in the Swift language plugin
due to the language's dynamic nature.

rdar://problem/57756763

Differential Revision: https://reviews.llvm.org/D71233