AArch64AsmParser: Restore Lsym@page-offset support

https://github.com/llvm/llvm-project/pull/134202 removed support for
`sym@page-offset` in instruction operands. This change is generally
reasonable since subtracting an offset from a symbol typically doesn’t
make sense for Mach-O due to its .subsections_via_symbols mechanism, which treats
them as separate atoms.

However, BoringSSL relies on a temporary symbol with a negative offset,
which can be meaningful when the symbol and the referenced location are
within the same atom.
```
../../third_party/boringssl/src/gen/bcm/p256-armv8-asm-apple.S:1160:25: error: unexpected token in argument list
 adrp x23,Lone_mont@PAGE-64
```

It's worth noting that expressions involving @ can be complex and
brittle in MCParser, and much of the Mach-O @ offsets remains
under-tested.

* Allow default argument for parsePrimaryExpr. The argument, used by the niche llvm-ml,
  should not require other targets to adapt.

llvm/include/llvm/MC/MCParser/MCAsmParser.h

@@ -308,7 +308,7 @@ public:
   /// on error.
   /// \return - False on success.
   virtual bool parsePrimaryExpr(const MCExpr *&Res, SMLoc &EndLoc,
-                                AsmTypeInfo *TypeInfo) = 0;
+                                AsmTypeInfo *TypeInfo = nullptr) = 0;
 
   /// Parse an arbitrary expression, assuming that an initial '(' has
   /// already been consumed.

llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp

@@ -4483,12 +4483,16 @@ bool AArch64AsmParser::parseSymbolicImmVal(const MCExpr *&ImmVal) {
     if (getParser().parseAtSpecifier(ImmVal, EndLoc))
       return true;
     const MCExpr *Term;
-    if (parseOptionalToken(AsmToken::Plus)) {
-      if (getParser().parseExpression(Term, EndLoc))
-        return true;
-      ImmVal =
-          MCBinaryExpr::create(MCBinaryExpr::Add, ImmVal, Term, getContext());
-    }
+    MCBinaryExpr::Opcode Opcode;
+    if (parseOptionalToken(AsmToken::Plus))
+      Opcode = MCBinaryExpr::Add;
+    else if (parseOptionalToken(AsmToken::Minus))
+      Opcode = MCBinaryExpr::Sub;
+    else
+      return false;
+    if (getParser().parsePrimaryExpr(Term, EndLoc))
+      return true;
+    ImmVal = MCBinaryExpr::create(Opcode, ImmVal, Term, getContext());
   }
 
   return false;
@@ -5026,11 +5030,15 @@ bool AArch64AsmParser::parseOperand(OperandVector &Operands, bool isCondCode,
       return true;
     if (getParser().parseAtSpecifier(IdVal, E))
       return true;
-    if (parseOptionalToken(AsmToken::Plus)) {
-      if (getParser().parseExpression(Term, E))
+    std::optional<MCBinaryExpr::Opcode> Opcode;
+    if (parseOptionalToken(AsmToken::Plus))
+      Opcode = MCBinaryExpr::Add;
+    else if (parseOptionalToken(AsmToken::Minus))
+      Opcode = MCBinaryExpr::Sub;
+    if (Opcode) {
+      if (getParser().parsePrimaryExpr(Term, E))
         return true;
-      IdVal =
-          MCBinaryExpr::create(MCBinaryExpr::Add, IdVal, Term, getContext());
+      IdVal = MCBinaryExpr::create(*Opcode, IdVal, Term, getContext());
     }
     Operands.push_back(AArch64Operand::CreateImm(IdVal, S, E, getContext()));
 

llvm/test/MC/AArch64/arm64-diags.s

@@ -366,6 +366,9 @@ subs x20, x30, sym@PAGEOFF
 ; CHECK-ERRORS: subs x20, x30, sym@PAGEOFF
 ; CHECK-ERRORS:                ^
 
+add w3, w5, sym@PAGEOFF - 3-2
+; CHECK-ERRORS: [[#@LINE-1]]:28: error: unexpected token in argument list
+
 tbl v0.8b, { v1 }, v0.8b
 tbl v0.16b, { v1.8b, v2.8b, v3.8b }, v0.16b
 tbx v3.16b, { v12.8b, v13.8b, v14.8b }, v6.8b

llvm/test/MC/MachO/AArch64/darwin-ARM64-reloc.s

@@ -17,9 +17,13 @@ _fred:
         ldr	w2, [x3, _data_ext@gotpageoff]
         adrp    x0, L_.str@PAGE
 
+        adrp    x3, L1@PAGE - 5
+        add	x3, x3, L1@pageoff - (4+1)
+
 	.data
 _data:
         .quad _foo
+L1:
         .quad _foo + 4
         .quad _foo - _bar
         .quad _foo - _bar + 4
@@ -36,6 +40,34 @@ L_.str:
 
 ; CHECK:     Relocations [
 ; CHECK-NEXT:  Section __text {
+; CHECK-NEXT:     Relocation {
+; CHECK-NEXT:       Offset: 0x2C
+; CHECK-NEXT:       PCRel: 0
+; CHECK-NEXT:       Length: 2
+; CHECK-NEXT:       Type: ARM64_RELOC_ADDEND (10)
+; CHECK-NEXT:       Section: __cstring (3)
+; CHECK-NEXT:     }
+; CHECK-NEXT:     Relocation {
+; CHECK-NEXT:       Offset: 0x2C
+; CHECK-NEXT:       PCRel: 0
+; CHECK-NEXT:       Length: 2
+; CHECK-NEXT:       Type: ARM64_RELOC_PAGEOFF12 (4)
+; CHECK-NEXT:       Symbol: _data (2)
+; CHECK-NEXT:     }
+; CHECK-NEXT:     Relocation {
+; CHECK-NEXT:       Offset: 0x28
+; CHECK-NEXT:       PCRel: 0
+; CHECK-NEXT:       Length: 2
+; CHECK-NEXT:       Type: ARM64_RELOC_ADDEND (10)
+; CHECK-NEXT:       Section: __cstring (3)
+; CHECK-NEXT:     }
+; CHECK-NEXT:     Relocation {
+; CHECK-NEXT:       Offset: 0x28
+; CHECK-NEXT:       PCRel: 1
+; CHECK-NEXT:       Length: 2
+; CHECK-NEXT:       Type: ARM64_RELOC_PAGE21 (3)
+; CHECK-NEXT:       Symbol: _data (2)
+; CHECK-NEXT:     }
 ; CHECK-NEXT:    Relocation {
 ; CHECK-NEXT:       Offset: 0x24
 ; CHECK-NEXT:       PCRel: 1