Add @compiles builtin

[marler8997]

@compiles(expr: var) bool

Takes an expression and returns a bool indicating whether the expression compiles.

This can be used to create test failure cases or implement sophisticated compile-time logic for fine-grained control over "generics" specialization.

Dlang equivalent __traits(compiles, <EXPR>) (https://dlang.org/spec/traits.html#compiles)

Example: used for test failure cases

pub fn foo(comptime T: type) type {
    switch (T) {
        builtin.TypeInfo.Pointer => { return T; }
        else => @compileError("foo requires Pointer types but got " ++ @typeName(T));
    }
}

test "foo" {
    // make sure this fails
    std.testing.expect(!@compiles(foo(u8)));
}

[marler8997]

Duplicate of #513