Adds explicit overload for `ExpressibleByStringLiteral` syntax types

### Description

Nowadays we have `DeclSyntax`, `ExprSyntax`, `StmtSyntax`, `TokenSyntax`, `TypeSyntax`... with `ExpressibleByStringLiteral` conformance. If an initializer takes a concrete-typed parameter of one of these types, string literal syntax works perfectly, reducing lots of boilerplate codes.

However, for initializers with generic parameters like ``, such syntax won't work and we need to explicitly express out `ExprSyntax("...")` or `"..." as ExprSyntax`. Adding an overload with concrete type `ExprSyntax` can solve the problem, making `SwiftSyntaxBuilder` easier to use.