[MC] Recursively calculate symbol offset

This is speculative since I'm not sure if there's some implicit contract that a
variable symbol must not have another variable symbol in its evaluation tree.

Downstream bug: https://bugs.chromium.org/p/chromium/issues/detail?id=471146#c23.

Test is based on alias.s (removed checks since we just need to know it didn't
crash).

Differential Revision: https://reviews.llvm.org/D109109

llvm/lib/MC/MCFragment.cpp

@@ -128,15 +128,19 @@ static bool getSymbolOffsetImpl(const MCAsmLayout &Layout, const MCSymbol &S,
   const MCSymbolRefExpr *A = Target.getSymA();
   if (A) {
     uint64_t ValA;
-    if (!getLabelOffset(Layout, A->getSymbol(), ReportError, ValA))
+    // FIXME: On most platforms, `Target`'s component symbols are labels from
+    // having been simplified during evaluation, but on Mach-O they can be
+    // variables due to PR19203. This, and the line below for `B` can be
+    // restored to call `getLabelOffset` when PR19203 is fixed.
+    if (!getSymbolOffsetImpl(Layout, A->getSymbol(), ReportError, ValA))
       return false;
     Offset += ValA;
   }
 
   const MCSymbolRefExpr *B = Target.getSymB();
   if (B) {
     uint64_t ValB;
-    if (!getLabelOffset(Layout, B->getSymbol(), ReportError, ValB))
+    if (!getSymbolOffsetImpl(Layout, B->getSymbol(), ReportError, ValB))
       return false;
     Offset -= ValB;
   }

llvm/test/MC/MachO/chained-alias-offset.s

@@ -0,0 +1,12 @@
+// RUN: llvm-mc -triple x86_64-apple-macos %s -filetype=obj | llvm-readobj --symbols - | FileCheck %s
+l_a:
+l_b = l_a + 1
+l_c = l_b
+        .long l_c
+
+// CHECK: Name: l_a
+// CHECK: Value: 0x0
+// CHECK: Name: l_b
+// CHECK: Value: 0x1
+// CHECK: Name: l_c
+// CHECK: Value: 0x1