Fall back to the AssignExprs startLoc when the EqualLoc is invalid

SIL Location diagnostics point at the getLoc, so when the EqualLoc is invalid we get no loc, while the startLoc is a fine alternative for these types of diagnostics.

Author: mxswd

include/swift/AST/Expr.h

@@ -4331,7 +4331,13 @@ class AssignExpr : public Expr {
 
   SourceLoc getEqualLoc() const { return EqualLoc; }
 
-  SourceLoc getLoc() const { return EqualLoc; }
+  SourceLoc getLoc() const {
+    SourceLoc loc = EqualLoc;
+    if (loc.isValid()) {
+      return loc;
+    }
+    return getStartLoc();
+  }
   SourceLoc getStartLoc() const {
     if (!isFolded()) return EqualLoc;
     return ( Dest->getStartLoc().isValid()