[gardening] Fix a vs an typos.

CHANGELOG.md

@@ -49,7 +49,7 @@ Swift 3.1
   the compiler will prevent them from being used in the first place.
 
 * Indirect fields from C structures and unions are now always imported, while
-  they previously weren't imported if they belonged to an union. This is done by
+  they previously weren't imported if they belonged to a union. This is done by
   naming anonymous fields. For example:
 
   ```c

include/swift/AST/ExprNodes.def

@@ -40,7 +40,7 @@
 #define UNCHECKED_EXPR(Id, Parent) EXPR(Id, Parent)
 #endif
 
-/// An literal expression node represents a literal value, such as a number,
+/// A literal expression node represents a literal value, such as a number,
 /// boolean, string, etc.
 ///
 /// By default, these are treated like any other expression.

lib/SILOptimizer/Analysis/EscapeAnalysis.cpp

@@ -1157,7 +1157,7 @@ bool EscapeAnalysis::buildConnectionGraphForDestructor(
     // The object is local, but we cannot determine its type.
     return false;
   }
-  // If Ty is a an optional, its deallocation is equivalent to the deallocation
+  // If Ty is an optional, its deallocation is equivalent to the deallocation
   // of its payload.
   // TODO: Generalize it. Destructor of an aggregate type is equivalent to calling
   // destructors for its components.

stdlib/public/runtime/StaticBinaryELF.cpp

@@ -121,7 +121,7 @@ public:
     if (programHeaders == nullptr) {
       return;
     }
-    // If a interpreter is set in the program headers then this is a
+    // If an interpreter is set in the program headers then this is a
     // dynamic executable and therefore not valid.
     for (size_t idx = 0; idx < elfHeader.e_phnum; idx++) {
       if (programHeaders[idx].p_type == PT_INTERP) {