This PR adds support in the shared CFG library for including parameters and their default values, and applies this to C# (even though, technically, default values are inlined into the callers).

We use Matching successors to denote whether an argument has been supplied or not. For example, this C# program

string M1(bool b, string s = "", int i = 0)
{
    return b + s + i;
}

gives rise to the following CFG (skipping the parts inside the body):

flowchart TD
1["Entry"]
10["After i [no-match]"]
11["i"]
12["0"]
13["{...}"]
4["b"]
5["After s [match]"]
6["After s [no-match]"]
7["s"]
8['""']
9["After i [match]"]

1 --> 4
4 --> 7
5 --> 11
6 --> 8
7 -- match --> 5
7 -- no-match --> 6
8 --> 11
9 --> 13
10 --> 12
11 -- match --> 9
11 -- no-match --> 10
12 --> 13

With parameters and their defaults added to the CFG, we replace existing SSA definitions at function entry with their CFG nodes (except when functions have multiple bodies), and we also add SSA definitions corresponding to defaults, which means that phi nodes will automatically be inserted.

Commit-by-commit review is strongly encouraged.