ParseOneInputFile allocates large amount of data on the LOH

[davkean]

Trace: https://developercommunity.visualstudio.com/content/problem/245786/slow-f-editing-experience-up-to-a-minute-until-typ.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	5196	1,652.0	744.6	8,424.125	40,245.7	29,166.1	0.7	1.026	7.7	18
0	3869	886.0	744.6	8,424.125	14,865.1	20,456.4	0.5	0.083	3.8	0
1	1286	53.5	744.6	3,284.108	17,473.7	8,524.0	0.6	0.746	13.6	0
2	41	1,652.0	744.6	502.652	7,906.8	185.7	0.3	7.355	192.8	18

14 MB on the LOH:

[cartermp]

Addressed by #6001