Type providers get created hundreds of times causing large LOH allocations and UI delays inside VS

[davkean]

Trace: https://developercommunity.visualstudio.com/content/problem/379653/vs-slow-typing-and-hang-1.html.

GC Rollup By Generation

Gen	Count	MaxPause	MaxPeak MB	Max AllocMB/sec	TotalPause	TotalAlloc MB	Alloc MB/MSec GC	Survived MB/MSec GC	MeanPause	Induced
ALL	305	13,100.2	2,515.3	9,389.575	23,078.5	2,845.1	0.1	13.147	75.7	1
0	187	20.6	2,481.5	4,736.269	1,031.3	1,109.3	0.4	0.050	5.5	0
1	108	13,100.2	2,509.7	9,389.575	14,950.0	1,486.1	5.3	0.199	138.4	0
2	10	728.8	2,515.3	14.396	7,097.2	249.7	2.5	20,674.062	709.7	1

In this trace, we're seeing large percentage of time spent collecting generations 1 & 2. If we dig into it, we can see that the majority of Gen2 collections are due to us creating hundreds of the same type providers over and over again, and they themselves allocating around 800KB each time. This results in about ~1.5 GB allocations all ending up on the LOH.

I would verify if we actually need to create type providers over and over again, and if not, introduce a cache based on the inputs so that we avoid this.

[davkean]

In total, this type provider path is allocating 2GB of data:

[davkean]

Note, I've filed #5931 to track the assembly metadata that the type providers themselves are loading.