Swift: fix bad join order in Pattern.getImmediateMatchingExpr

On Signal-iOS, this snippet:

```codeql
class Pattern extends Generated::Pattern {
  ...
  Expr getImmediateMatchingExpr() {
    ...
    exists(PatternBindingDecl v, int i |
      v.getPattern(i) = this and
      result = v.getInit(i)
    )
    ...
  }
  ...
}
```

Had the following join order:

```
             33926   ~0%    {3} r8 = SCAN PatternBindingDecl#ab5153b9::Generated::PatternBindingDecl::getImmediateInit#1#dispred#fff OUTPUT In.1, In.0, In.2
        2565045964   ~0%    {4} r9 = JOIN r8 WITH pattern_binding_decl_patterns_102#join_rhs ON FIRST 1 OUTPUT Lhs.1, Rhs.1, Lhs.2, Rhs.2
             33926   ~0%    {2} r10 = JOIN r9 WITH Synth#5f134a93::Synth::convertPatternBindingDeclToRaw#1#ff ON FIRST 2 OUTPUT Lhs.3, Lhs.2
             33926   ~2%    {2} r11 = JOIN r10 WITH Synth#5f134a93::Synth::convertPatternFromRaw#1#ff ON FIRST 1 OUTPUT Rhs.1, Lhs.1
             33926   ~1%    {2} r12 = JOIN r11 WITH Element#e67432df::Generated::Element::resolve#ff ON FIRST 1 OUTPUT Lhs.1, Rhs.1
             33926   ~4%    {2} r13 = JOIN r12 WITH Element#e67432df::Generated::Element::resolve#ff ON FIRST 1 OUTPUT Lhs.1, Rhs.1
```

After applying `pragma[only_bind_out]` to `this`:

```
        198815   ~1%    {2} r4 = SCAN Synth#5f134a93::Synth::TPattern#f OUTPUT In.0, In.0
        198815   ~0%    {2} r5 = JOIN r4 WITH Element#e67432df::Generated::Element::resolve#ff_10#join_rhs ON FIRST 1 OUTPUT Rhs.1, Lhs.1
         75626   ~0%    {3} r6 = JOIN r5 WITH PatternBindingDecl#ab5153b9::Generated::PatternBindingDecl::getImmediatePattern#1#dispred#fff_201#join_rhs ON FIRST 1 OUTPUT Rhs.1, Rhs.2, Lhs.1
         33926   ~1%    {2} r7 = JOIN r6 WITH PatternBindingDecl#ab5153b9::Generated::PatternBindingDecl::getImmediateInit#1#dispred#fff ON FIRST 2 OUTPUT Rhs.2, Lhs.2
         33926   ~4%    {2} r8 = JOIN r7 WITH Element#e67432df::Generated::Element::resolve#ff ON FIRST 1 OUTPUT Lhs.1, Rhs.1

```

Author: d10c

swift/ql/lib/codeql/swift/elements/pattern/Pattern.qll

@@ -37,7 +37,7 @@ class Pattern extends Generated::Pattern {
     )
     or
     exists(PatternBindingDecl v, int i |
-      v.getPattern(i) = this and
+      v.getPattern(i) = pragma[only_bind_out](this) and
       result = v.getInit(i)
     )
   }