Lifetimes: handle MutableSpan reassignment and 'inout' arguments

[atrick]

Fix miscompilations involving MutableSpan.

• [NFC] add a new entry point for DiagnoseDependence.reportError
  Allow diagnosing values that have no relevant users. Such as an @inout argument.

• [NFC] LifetimeDependenceDefUseWalker.inoutDependence entry point
  Handle cases where there's no relevant operand.

• [NFC] Add ApplySite.fullyAssigns(Operand) query

• Fix LifetimeDependenceDefUseWalker for @inout reassignment

• Extend AddressInitializationWalker.findSingleInitializer
  Handle applies that reassign the lifetime of their operand vs. the value of
  their operand.

• LocalVariableReachableAccess: handle lifetime reassignment
  A ~Escapable inout apply operand that does not propagate its own lifetime can be
  treated as being assigned a new lifetime in the caller.

  @Lifetime(arg: copy source)
  func foo<T: ~Escapable>(arg: inout T, source: T) {
  arg = source
  }

  A call to 'foo' initializes the lifetime of 'arg', thereby ending the lifetime
  of the previous value.

• [NFC] Rename ArgumentConventions.parameterIndex(ofArgumentIndex:)

• [NFC] Add ApplySite.parameterDependence(target:source:)

• Fix LifetimeDependenceDefUseWalker to follow inout dependence
  Fixes rdar://157796728 ([nonescapable] [miscompile] No diagnostic error when an
  inout MutableSpan is reassigned to a different lifetime source)

• [NFC] LocalVariableAccessInfo.description

• LocalVariableUtils add non-escaping closure capture support
  Don't always consider an inout_aliasable argument to have
  escaped. AccessEnforcementSelection has already done that analysis and left
  begin_access [dynamic] artifacts if the argument has escaped in any meaningful
  way. Use that information to

  Essential for supporting autoclosures that call mutating methods on span-like
  things. Such as UTF8Span.UnicodeScalarIterator.skipForward():

  e.g. while numSkipped < n && skipForward() != 0 { ... }

• [NFC] extend AddressInitializationWalker to report address reads

• Fix LocalVariableReachableAccess to handle potential reassignment.
  Define LocalAccessInfo._isFullyAssigned to mean that the access does not read
  the incoming value. Then treat any assignment that isn't full as a potential read.

• LocalVariableUtils: precisely handle function live-out
  Only record an outgoingArgument access when the current allocation is an
  outgoing argument and the function exiting instruction is ReturnInst.

  This avoids invalid SIL where we attempt to extend end_access up to an unwind but
  fail to actually materialize that end_access.

• Test lifetime reassignment

• Test noescape closure capture lifetimes

• Update tests for local variable reachability for debug output

• Update lifetime tests for improved diagnostics

[atrick]

@swift-ci test

[atrick]

@swift-ci test

[atrick]

@swift-ci test