[clang] [DebugInfo] Correct the line attribution for IF branches (PR #108300)

[Paul T Robinson via cfe-commits]

================
@@ -0,0 +1,45 @@
+// RUN: %clang_cc1 -debug-info-kind=limited -gno-column-info -triple=x86_64-pc-linux -emit-llvm %s -o - | FileCheck  %s
+
+// The important thing is that the compare and the conditional branch have
+// locs with the same scope (the lexical block for the 'if'). By turning off
+// column info, they end up with the same !dbg record, which halves the number
+// of checks to verify the scope.
+
+int c = 2;
+
+int f() {
+#line 100
+  if (int a = 5; a > c)
+    return 1;
+  return 0;
+}
+// CHECK-LABEL: define {{.*}} @_Z1fv()
+// CHECK:       = icmp {{.*}} !dbg [[F_CMP:![0-9]+]]
+// CHECK-NEXT:  br i1 {{.*}} !dbg [[F_CMP]]
----------------
pogo59 wrote:

I followed this where my test case led me. You're right it's worth looking at other control structures that have implied lexical blocks.

> & I guess this all only applies when the control structure doesn't have `{}`? (that being why the test doesn't have braces?)

No, this applies in all cases.
```
if (int a = 5; b > c) {
    return a;
} else {
    return a + 1;
}
```
The implied lexical block started by the `if` statement spans its then and else parts; whether those parts are simple or compound doesn't matter. Analogous to how a function definition has an implied lexical block at the opening paren of the parameter list; the function name is visible in the containing scope, the parameter names are not.

https://github.com/llvm/llvm-project/pull/108300