Base: macro b_str: restrict argument to String

[nsajko]

Should make the sysimage less vulnerable to invalidation.

As far as I understand this change can't break any code, because:

• the macro definition requires AbstractString
• the way Julia macros work, the argument is either a literal or an Expr
• String is the only AbstractString with literals

[nsajko]

Prevents the invalidation:

{
    "method_instance": {
        "method": "var\"@b_str\"(__source__::LineNumberNode, __module__::Module, s) @ Base strings/io.jl:588",
        "method_instance": "MethodInstance for var\"@b_str\"(::LineNumberNode, ::Module, ::Any)"
    },
    "children": [
    ]
}

Also fixing the following invalidation would cause a chain reaction where even more invalidation gets fixed (for a future PR to JuliaSyntax):

{
    "method_instance": {
        "method": "var\"@u8_str\"(__source__::LineNumberNode, __module__::Module, str) @ Base.JuliaSyntax /cache/build/builder-amdci5-7/julialang/julia-master/base/JuliaSyntax/src/utils.jl:54",
        "method_instance": "MethodInstance for Base.JuliaSyntax.var\"@u8_str\"(::LineNumberNode, ::Module, ::Any)"
    },
    "children": [
    ]
}