[scudo] Compute the default aligned pointer without tag

#83493 slightly
changed the order of computation of block addresses and
pointers, causing the value of DefaultAlignedPtr to
include the MTE tag. Move this computation earlier so it
matches the old behavior.

This fixes a UBSan failure in Trusty:
secure os: UBSan: (overflow:-) external/scudo/standalone/combined.h:1070:35
secure os: Details: unsigned integer overflow: 8988807738704 - 144124176883594576 cannot be represented in type 'uptr'

Author: ahomescu

compiler-rt/lib/scudo/standalone/combined.h

@@ -1052,6 +1052,10 @@ class Allocator {
                                 void *Block, const uptr UserPtr,
                                 const uptr SizeOrUnusedBytes,
                                 const FillContentsMode FillContents) {
+    // Compute the default pointer before adding the header tag
+    const uptr DefaultAlignedPtr =
+        reinterpret_cast<uptr>(Block) + Chunk::getHeaderSize();
+
     Block = addHeaderTag(Block);
     // Only do content fill when it's from primary allocator because secondary
     // allocator has filled the content.
@@ -1064,8 +1068,6 @@ class Allocator {
 
     Chunk::UnpackedHeader Header = {};
 
-    const uptr DefaultAlignedPtr =
-        reinterpret_cast<uptr>(Block) + Chunk::getHeaderSize();
     if (UNLIKELY(DefaultAlignedPtr != UserPtr)) {
       const uptr Offset = UserPtr - DefaultAlignedPtr;
       DCHECK_GE(Offset, 2 * sizeof(u32));
@@ -1096,6 +1098,10 @@ class Allocator {
     const Options Options = Primary.Options.load();
     DCHECK(useMemoryTagging<AllocatorConfig>(Options));
 
+    // Compute the default pointer before adding the header tag
+    const uptr DefaultAlignedPtr =
+        reinterpret_cast<uptr>(Block) + Chunk::getHeaderSize();
+
     void *Ptr = reinterpret_cast<void *>(UserPtr);
     void *TaggedPtr = Ptr;
 
@@ -1194,8 +1200,6 @@ class Allocator {
 
     Chunk::UnpackedHeader Header = {};
 
-    const uptr DefaultAlignedPtr =
-        reinterpret_cast<uptr>(Block) + Chunk::getHeaderSize();
     if (UNLIKELY(DefaultAlignedPtr != UserPtr)) {
       const uptr Offset = UserPtr - DefaultAlignedPtr;
       DCHECK_GE(Offset, 2 * sizeof(u32));