[Diagnostics] Improve diagnostics for optional/integer as boolean condition

[owenv]

let x: Int = 0
if x {} // 1
if !x {} // 2

let y: String? = nil
if y {} // 3
if !y {} // 4

Used to give a good diagnostic for 3 and an ok one for 4. Now it results in the following:

hello.swift:3:4: error: type 'Int' cannot be used as a boolean; test for '!= 0' instead
if x {}
   ^
   ( != 0)
hello.swift:4:5: error: type 'Int' cannot be used as a boolean; test for '== 0' instead
if !x {}
   ~^
    ( == 0)
hello.swift:8:4: error: optional type 'String?' cannot be used as a boolean; test for '!= nil' instead 
if y {}
   ^
   ( != nil)
hello.swift:9:5: error: optional type 'String?' cannot be used as a boolean; test for '== nil' instead
if !y {}
   ~^
    ( == nil)

Resolves a FIXME and SR-4932

[owenv]

@swift-ci please smoke test

[owenv]

@swift-ci please smoke test

[xedin]

Great, thank you!