[String] Scalar-alignment bug fixes.

Fixes a general category (pun intended) of scalar-alignment bugs
surrounding exchanging non-scalar-aligned indices between views and
for slicing.

SE-0180 unifies the Index type of String and all its views and allows
non-scalar-aligned indices to be used across views. In order to
guarantee behavior, we often have to check and perform scalar
alignment. To speed up these checks, we allocate a bit denoting
known-to-be-aligned, so that the alignment check can skip the
load. The below shows what views need to check for alignment before
they can operate, and whether the indices they produce are aligned.

┌───────────────╥────────────────────┬──────────────────────────┐
│ View          ║ Requires Alignment │ Produces Aligned Indices │
╞═══════════════╬════════════════════╪══════════════════════════╡
│ Native UTF8   ║ no                 │ no                       │
├───────────────╫────────────────────┼──────────────────────────┤
│ Native UTF16  ║ yes                │ no                       │
╞═══════════════╬════════════════════╪══════════════════════════╡
│ Foreign UTF8  ║ yes                │ no                       │
├───────────────╫────────────────────┼──────────────────────────┤
│ Foreign UTF16 ║ no                 │ no                       │
╞═══════════════╬════════════════════╪══════════════════════════╡
│ UnicodeScalar ║ yes                │ yes                      │
├───────────────╫────────────────────┼──────────────────────────┤
│ Character     ║ yes                │ yes                      │
└───────────────╨────────────────────┴──────────────────────────┘

The "requires alignment" applies to any operation taking a
String.Index that's not defined entirely in terms of other operations
taking a String.Index. These include:

* index(after:)
* index(before:)
* subscript
* distance(from:to:) (since `to` is compared against directly)
* UTF16View._nativeGetOffset(for:)

stdlib/public/core/StringUnicodeScalarView.swift

@@ -106,22 +106,29 @@ extension String.UnicodeScalarView: BidirectionalCollection {
   /// - Precondition: The next location exists.
   @inlinable @inline(__always)
   public func index(after i: Index) -> Index {
+    let i = _guts.scalarAlign(i)
     _internalInvariant(i < endIndex)
     // TODO(String performance): isASCII fast-path
 
     if _fastPath(_guts.isFastUTF8) {
       let len = _guts.fastUTF8ScalarLength(startingAt: i._encodedOffset)
-      return i.encoded(offsetBy: len)
+      return i.encoded(offsetBy: len).aligned
     }
 
     return _foreignIndex(after: i)
   }
 
+  @_alwaysEmitIntoClient // Swift 5.1 bug fix
+  public func distance(from start: Index, to end: Index) -> Int {
+    return _distance(from: _guts.scalarAlign(start), to: _guts.scalarAlign(end))
+  }
+
   /// Returns the previous consecutive location before `i`.
   ///
   /// - Precondition: The previous location exists.
   @inlinable @inline(__always)
   public func index(before i: Index) -> Index {
+    let i = _guts.scalarAlign(i)
     precondition(i._encodedOffset > 0)
     // TODO(String performance): isASCII fast-path
 
@@ -130,7 +137,7 @@ extension String.UnicodeScalarView: BidirectionalCollection {
         return _utf8ScalarLength(utf8, endingAt: i._encodedOffset)
       }
       _internalInvariant(len <= 4, "invalid UTF8")
-      return i.encoded(offsetBy: -len)
+      return i.encoded(offsetBy: -len).aligned
     }
 
     return _foreignIndex(before: i)
@@ -412,7 +419,7 @@ extension String.UnicodeScalarView {
     let cu = _guts.foreignErrorCorrectedUTF16CodeUnit(at: i)
     let len = UTF16.isLeadSurrogate(cu) ? 2 : 1
 
-    return i.encoded(offsetBy: len)
+    return i.encoded(offsetBy: len).aligned
   }
 
   @usableFromInline @inline(never)
@@ -423,6 +430,6 @@ extension String.UnicodeScalarView {
     let cu = _guts.foreignErrorCorrectedUTF16CodeUnit(at: priorIdx)
     let len = UTF16.isTrailSurrogate(cu) ? 2 : 1
 
-    return i.encoded(offsetBy: -len)
+    return i.encoded(offsetBy: -len).aligned
   }
 }