[AST] Use explicit type erasure in TypeSourceInfo constructor (#68435)

When this file is included in a project compiled with GCC 13 `-Werror`,
compilation fails with the following diagnostic:
<pre>
/usr/lib/llvm-17.0/include/clang/AST/TypeLoc.h: In constructor
'clang::TypeSourceInfo::TypeSourceInfo(clang::QualType, size_t)':
/usr/lib/llvm-17.0/include/clang/AST/TypeLoc.h:245:9: error: 'void*
memset(void*, int, size_t)' clearing an object of non-trivial type
'class clang::TypeSourceInfo'; use assignment instead
[-Werror=class-memaccess]
  245 |   memset(this + 1, 0, DataSize);
      |   ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~

</pre>
To avoid this, we add an explicit type cast. The cast to `void*` makes
sense, since other member functions of `TypeSourceInfo` also treat the
buffer `(this + 1)` of length `DataSize` as opaque memory, and was
likely left out by mistake.

Fixes: 4498663f ("[AST] Initialized data after TypeSourceInfo")

I also suggest to apply this to release/17.x if possible.