Issue Description:

When parsing some source code that containing several "[", it may have performance issue while use significant resources. For example, when running the following code snippets below, swiftc may just hang there and the computer may lag very much when the situation lasts longer (after one hour).

# h_01.swift
func myfunc() -> Bool {
 return [[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[ == 1
}

# h_02.swift
func myfunc() {
 [[[[[[[[[[[[[[[[[[[[[_ = Selector("")
}

# h_03.swift
func myfunc() {
 [[[[[[[[[[[[[[[[[[[[[[ + ""
}

# h_04.swift
func myfunc() {
 [[[[[[[[[[[[[[[[[[[[[[ | wrong
}