Implement SE-0001: Allow (most) keywords as argument labels.

Allow all keywords except for parameter introducers (var/let/inout) to
be argument labels when declaring or calling a
function/initializer/subscript, e.g., this

  func touchesMatching(phase: NSTouchPhase, `in` view: NSView?) -> Set<NSTouch>

can now be expressed as

  func touchesMatching(phase: NSTouchPhase, in view: NSView?) -> Set<NSTouch>

and the call goes from

  event.touchesMatching(phase, `in`: view)

to

  event.touchesMatching(phase, in: view)

Fixes [SR-344](https://bugs.swift.org/browse/SR-344) /
rdar://problem/22415674.

lib/Basic/StringExtras.cpp

@@ -27,6 +27,13 @@
 using namespace swift;
 using namespace camel_case;
 
+bool swift::canBeArgumentLabel(StringRef identifier) {
+  if (identifier == "var" || identifier == "let" || identifier == "inout")
+    return false;
+
+  return true;
+}
+
 PrepositionKind swift::getPrepositionKind(StringRef word) {
 #define DIRECTIONAL_PREPOSITION(Word)           \
   if (word.equals_lower(#Word))                 \