[Matrix] Define Behavior of a new MatrixSwizzleExpr ASTNode

[farzonl]

We need a means of supporting ._m and ._ chains so we can swizzle matrix elements
Like so:

export void fn() {
    float2x2 M = {1,2,3,4};
    float2 V = M._m00_m01; // produces a vector.
    M._m10_m01 = 1.xx; // prodces a swizzled vector l-value!
}

The new ASTNode would need to:

• retain source information accurately enough to drive AST-driven tooling.
• needs to be a single l-value of vector type that represents elements arbitrarily dispersed across the matrix.