Modified C# directive parser to allow #pragma warning disable/restore to take string IDs in addition to integers in order to generalize to user-generated diagnostics. Compiler-generated diagnostics all have integer error codes in addition to a string ID, but user-generated diagnostics are only identified by a string ID. This change also affects the language grammar, effectively changing the grammar of "warning-list" in section 2.5.8.1 of the C# spec from:

warning-list:
    decimal-digits
    warning-list   whitespaceopt   ,   whitespaceopt   decimal-digits

to:

warning-id:
    decimal-digits
    string-literal

warning-list:
    warning-id
    warning-list   whitespaceopt   ,   whitespaceopt   warning-id

As implemented, the compiler will accept the following:

#pragma warning disable ?CS0168?
#pragma warning disable ?CS0168?, ?CS0219?
#pragma warning disable ?CS0168?, 219 // , ... etc.

And warn about the following:

#pragma warning disable ?CS168? // warning CS1691: 'CS168' is not a valid warning number
#pragma warning disable 1 // warning CS1691: ?1? is not a valid warning number (current behavior)

Only strings that match 'CS[0-9]*' are assumed to be compiler diagnostic IDs, so the compiler will not warn about something like:

#pragma warning disable "CS1MyWarning" // This could be a user diagnostic ID (changeset 1209083)