Silence Swift compiler warnings in Swift runtime

[niw]

Problems

Current Swift runtime contains multiple usage of var which is not necessary and recent Swift compiler emits warning for such var usage.
For example:

antlr/antlr4/runtime/Swift/Sources/Antlr4/Lexer.swift:135:13: warning: variable 'tokenStartMarker' was never mutated; consider changing to 'let' constant
        var tokenStartMarker = _input.mark()
        ~~~ ^
        let

Solution
Since there is no easy way to silence it from compiler option especially when it provided in Swift Package Manager,
Fix these usage of var instead of let causes Swift compiler warnings.

[hanjoes]

will tryy to merge it as part of #3301