[scudo] Change overloaded function name append.

The ScopedString class has two functions named append. One takes
a va_list, but on some platforms va_list is typedef'd to char*.
That means that this call:

  std::string value;
  Str.append("print this string %s", value.c_str());

The compiler can incorrectly think this is the va_list function,
leading to crashes when calling this. To fix this, change the name
of the va_list function to be vappend to avoid this.

Fix https://github.com/llvm/llvm-project/issues/62893

Reviewed By: Chia-hungDuan

Differential Revision: https://reviews.llvm.org/D153389