Fix mscordbi metadata reader alignment bug on Linux. (#19196)

Works fine on Windows minidumps, but on Linux (via OpenVirtualProcess
for production breakpoints and future core dumps) the compiler's struct
alignment rules are different. On Windows, classes/structs are aligned
based on the largest field. On Linux, they are 4 byte aligned regardless
of the field sizes.

https://github.com/dotnet/coreclr/issues/17692

Author: Mike McLaughlin

src/md/datasource/datatargetreader.cpp

@@ -163,7 +163,13 @@ void DataTargetReader::Align(DWORD alignmentBytes)
 
 void DataTargetReader::AlignBase()
 {
+#ifdef _MSC_VER
+    // Windows MSVC compiler aligns structs based on the largest field size
     Align(m_currentStructureAlign);
+#else
+    // clang (on all platforms) aligns structs always on 4 byte boundaries
+    Align(4);
+#endif
 }
 
 HRESULT DataTargetReader::GetRemotePointerSize(ULONG32* pPointerSize)

src/md/datasource/targettypes.cpp

@@ -134,9 +134,12 @@ m_tableMax(0)
 HRESULT Target_MapSHash::ReadFrom(DataTargetReader & reader)
 {
     HRESULT hr = S_OK;
+    // Only the Windows MSVC compiler does this; not clang on Linux
+#ifdef _MSC_VER
     IfFailRet(reader.Skip8()); // this byte gets used by the base class even though it has no members
                                // I'm guessing this is so the 2nd base class (noncopyable) doesn't start at the same
                                // location, but I can't be sure. Whatever the cause, the layout skips a byte.
+#endif // _MSC_VER
     IfFailRet(reader.ReadPointer(&m_table));
     IfFailRet(reader.Read32(&m_tableSize));
     IfFailRet(reader.Read32(&m_tableCount));