[Responsiveness] CommandLineArguments.Parse substrings every argument twice causing lots of unneeded allocations (1% - 5%) during solution load

**Version Used**: 
17.0.0 Preview 1.0 [31313.406.main]

**Steps to Reproduce**:

CommandLineArguments.Parse is called _a lot_ during solution open by both Roslyn and the project system.

[Trace](https://microsoft-my.sharepoint.com/personal/paulv_microsoft_com/_layouts/15/onedrive.aspx?originalPath=aHR0cHM6Ly9taWNyb3NvZnQtbXkuc2hhcmVwb2ludC5jb20vOmY6L3AvcGF1bHYvRXRUY1d3X0FBb1pLc29WMVl5ajdUV2NCNEtIU1hkRWs1TTktd2JKMUhKWXpGQT9ydGltZT1wdGpQZlBFYjJVZw&id=%2Fpersonal%2Fpaulv%5Fmicrosoft%5Fcom%2FDocuments%2Fscale%2Ftraces%2F17%2E0%2E0%20Preview%201%2E0%20%5B31313%2E406%2Emain%5D%2Forchardcore%2Drc2%2FSolution%5F001%5F0156%5FGeneral%5Fnocache)

![image](https://user-images.githubusercontent.com/1103906/119065792-ab909b00-ba21-11eb-9bf1-89c20302a767.png)

This code double-parses every option twice just to find the ruleset option: https://github.com/dotnet/roslyn/blob/31103b6174bc95de900fe3f2fa05b3b2dff66437/src/Compilers/CSharp/Portable/CommandLine/CSharpCommandLineParser.cs#L143-L146

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[Responsiveness] CommandLineArguments.Parse substrings every argument twice causing lots of unneeded allocations (1% - 5%) during solution load #53570

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	foreach (string arg in flattenedArgs)
	{
	string? name, value;
	if (TryParseOption(arg, out name, out value) && (name == "ruleset"))

[Responsiveness] CommandLineArguments.Parse substrings every argument twice causing lots of unneeded allocations (1% - 5%) during solution load #53570

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