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Issue Description:

When compiling this file:

// Does not have to be a function; I first saw this with String(reflecting:).
func take(_: CustomStringConvertible) {}

// Must be in a function.
func wat() {
  // Must be an existential; Any works too.
  let value: CustomStringConvertible
  
  // Must be initialized on a separate line.
  value = "x"   
  
  // Must be in a closure and must be immediately called.
  ({ take(value) })()
}

Swift 4, with PR #8366 applied to avoid a crash, complains:

capturetest.swift:13:4: error: immutable value 'value' may not be passed inout
  ({ take(value) })()
   ^

This is at least better than Swift 3.1, which goes into an infinite loop. (That's with the parentheses around the closure; without them, it mis-parses the expression entirely.)

Examining the SIL, it looks like something decides that `value` needs to be passed to `take(_🙂` as `@lvalue_aliasable`, and the DI lifetime checker accurately complains that you can't do that to a `let` variable.