NuGet.ContentModel consumes too much memory for string operations

[jainaashish]

internal bug# 385896

NuGet.contentmodel allocates ~500mb out of 1.3gb string allocation for a customer scenario as mentioned in the internal bug.

Some of the stack trace from PerfView:

Name Inc % Inc
Type System.String 20.5 1,338,334,336

• clr!? 20.5 1,338,334,208
• mscorlib.ni 20.0 1,305,615,104
  |+ nuget.contentmodel 4.1 269,299,840
  |+ ?!? 1.9 122,823,536
  |+ microsoft.build.ni!Microsoft.Build.Shared.FileUtilities+ItemSpecModifiers.GetItemSpecModifier(System.String, System.String, System.String, System.String, System.String ByRef) 1.7 107,955,696

Pointers in code to look at some of string manipulations:

https://github.com/NuGet/NuGet.Client/blob/dev/src/NuGet.Core/NuGet.ContentModel/Infrastructure/Parser.cs#L26
https://github.com/NuGet/NuGet.Client/blob/dev/src/NuGet.Core/NuGet.ContentModel/Infrastructure/Parser.cs#L99
https://github.com/NuGet/NuGet.Client/blob/dev/src/NuGet.Core/NuGet.ContentModel/Infrastructure/Parser.cs#L137

[jainaashish]

It's fixed as part of NuGet/NuGet.Client#1590