[TLS] Add a uText to our TLS

ICU's uText APIs offer a cross-encoding means of interacting with
grapheme breaking. Storing one on our TLS allows us to explore mised
encodings, and even for UTF-16 might be profitable if setText
mallocs/frees an internal uText (which seems to be the case). This
isn't hooked up to anything yet, but its highly likely we will be
using it soon and adding it now enables more rapid development of the
UTF-8 prototype, due to it's tie-in with the runtime.

Author: milseman

stdlib/public/core/ThreadLocalStorage.swift

@@ -38,6 +38,7 @@ internal struct _ThreadLocalStorage {
   //
   // private
   internal var uBreakIterator: OpaquePointer
+  internal var uText: OpaquePointer
 
   // TODO: Consider saving two, e.g. for character-by-character comparison
 
@@ -55,8 +56,9 @@ internal struct _ThreadLocalStorage {
   // TODO: unowned reference to string owner, base address, and _countAndFlags
 
   // private: Should only be called by _initializeThreadLocalStorage
-  internal init(_uBreakIterator: OpaquePointer) {
+  internal init(_uBreakIterator: OpaquePointer, _uText: OpaquePointer) {
     self.uBreakIterator = _uBreakIterator
+    self.uText = _uText
   }
 
   // Get the current thread's TLS pointer. On first call for a given thread,
@@ -104,19 +106,26 @@ internal func _destroyTLS(_ ptr: UnsafeMutableRawPointer?) {
 internal func _createThreadLocalStorage()
   -> UnsafeMutablePointer<_ThreadLocalStorage>
 {
-  // Create and initialize one.
+  // Allocate and initialize a UBreakIterator and UText.
   var err = __swift_stdlib_U_ZERO_ERROR
   let newUBreakIterator = __swift_stdlib_ubrk_open(
       /*type:*/ __swift_stdlib_UBRK_CHARACTER, /*locale:*/ nil,
       /*text:*/ nil, /*textLength:*/ 0, /*status:*/ &err)
   _precondition(err.isSuccess, "Unexpected ubrk_open failure")
 
+  // utext_openUTF8 needs a valid pointer, even though we won't read from it
+  var a: Int8 = 0x41
+  let newUText = __swift_stdlib_utext_openUTF8(
+      /*ut:*/ nil, /*s:*/ &a, /*len:*/ 1, /*status:*/ &err)
+
+  _precondition(err.isSuccess, "Unexpected utext_openUTF8 failure")
+
   let tlsPtr: UnsafeMutablePointer<_ThreadLocalStorage>
     = UnsafeMutablePointer<_ThreadLocalStorage>.allocate(
       capacity: 1
   )
-  tlsPtr.initialize(
-    to: _ThreadLocalStorage(_uBreakIterator: newUBreakIterator)
-  )
+  tlsPtr.initialize(to: _ThreadLocalStorage(
+    _uBreakIterator: newUBreakIterator, _uText: newUText))
+
   return tlsPtr
 }