C++: Fix join-order problem with isBefore

Reported here: #17743

Without this change on the query provided by the user:
```
[2025-02-25 12:42:01] Evaluated non-recursive predicate quickquery::UnrealFunctionAnnotation.annotates/1#dispred#9cd6c269@c668c8tv in 23846ms (size: 20381473).
Evaluated relational algebra for predicate quickquery::UnrealFunctionAnnotation.annotates/1#dispred#9cd6c269@c668c8tv with tuple counts:
                 1   ~0%    {0} r1 = CONSTANT()[]
             27323   ~0%    {2}    | JOIN WITH `Location::Location.getEndLine/0#dispred#83af84ae#bf` CARTESIAN PRODUCT OUTPUT Rhs.0, Rhs.1
        6162566035   ~0%    {4}    | JOIN WITH `Location::Location.getStartLine/0#d54f9e6c` CARTESIAN PRODUCT OUTPUT Lhs.0, Lhs.1, Rhs.0, Rhs.1
                            {4}    | REWRITE WITH TEST InOut.1 < InOut.3
        3894825644   ~5%    {2}    | SCAN OUTPUT In.2, In.0
          73148692   ~0%    {3}    | JOIN WITH fun_decls_40#join_rhs ON FIRST 1 OUTPUT Lhs.1, Lhs.0, Rhs.1
          73148692   ~0%    {4}    | JOIN WITH `Location::Location.getFile/0#dispred#d1f8b5d1` ON FIRST 1 OUTPUT Lhs.1, Rhs.1, Lhs.0, Lhs.2
            864579   ~0%    {2}    | JOIN WITH `Location::Location.getFile/0#dispred#d1f8b5d1` ON FIRST 2 OUTPUT Lhs.2, Lhs.3
          13010742   ~1%    {2}    | JOIN WITH macroinvocations_20#join_rhs ON FIRST 1 OUTPUT Rhs.1, Lhs.1
          20653781   ~0%    {3}    | JOIN WITH `Macro::MacroAccess.getOutermostMacroAccess/0#d58b05db_10#join_rhs` ON FIRST 1 OUTPUT Rhs.1, _, Lhs.1
          20653781   ~4%    {3}    | REWRITE WITH Out.1 := 1
          20381473   ~8%    {2}    | JOIN WITH macroinvocations_03#join_rhs ON FIRST 2 OUTPUT Lhs.0, Lhs.2
                            return r1
```

With this change:
```
[2025-02-25 12:43:10] Evaluated non-recursive predicate quickquery::UnrealFunctionAnnotation.annotates/1#dispred#9cd6c269@11bf8956 in 928ms (size: 20381473).
Evaluated relational algebra for predicate quickquery::UnrealFunctionAnnotation.annotates/1#dispred#9cd6c269@11bf8956 with tuple counts:
            6873   ~3%    {2} r1 = SCAN fun_decls OUTPUT In.4, In.0
            6857   ~0%    {3}    | JOIN WITH `Location::Location.getStartLine/0#d54f9e6c` ON FIRST 1 OUTPUT Lhs.0, Lhs.1, Rhs.1
            6857   ~2%    {3}    | JOIN WITH `Location::Location.getFile/0#dispred#d1f8b5d1` ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.2
         6193961   ~0%    {3}    | JOIN WITH `Location::Location.getFile/0#dispred#d1f8b5d1_10#join_rhs` ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.2
        27389714   ~1%    {4}    | JOIN WITH macroinvocations_20#join_rhs ON FIRST 1 OUTPUT Lhs.0, Lhs.1, Lhs.2, Rhs.1
        27389714   ~1%    {4}    | JOIN WITH locations_default ON FIRST 1 OUTPUT Lhs.1, Lhs.2, Lhs.3, Rhs.4
                          {4}    | REWRITE WITH TEST InOut.3 < InOut.1
        13010742   ~1%    {2}    | SCAN OUTPUT In.2, In.0
        20653781   ~0%    {3}    | JOIN WITH `Macro::MacroAccess.getOutermostMacroAccess/0#d58b05db_10#join_rhs` ON FIRST 1 OUTPUT Rhs.1, _, Lhs.1
        20653781   ~4%    {3}    | REWRITE WITH Out.1 := 1
        20381473   ~8%    {2}    | JOIN WITH macroinvocations_03#join_rhs ON FIRST 2 OUTPUT Lhs.0, Lhs.2
                          return r1
```

Author: jketema

cpp/ql/lib/semmle/code/cpp/Location.qll

@@ -75,7 +75,10 @@ class Location extends @location {
 
   /** Holds if `this` comes on a line strictly before `l`. */
   pragma[inline]
-  predicate isBefore(Location l) { this.isBefore(l, false) }
+  predicate isBefore(Location l) {
+    this.getFile() = l.getFile() and
+    this.getEndLine() < l.getStartLine()
+  }
 
   /**
    * Holds if `this` comes strictly before `l`. The boolean `sameLine` is