generate a name of an unnamed global variable for Instruction Selection (#78293)

The goal of this PR is to fix the issue of global unnamed variables
causing SPIR-V Backend code generation to crash:
https://github.com/llvm/llvm-project/issues/78278

The reason for the crash is that GlobalValue's getGlobalIdentifier()
would fail for unnamed global variable when trying to access the first
character of the name (see lib/IR/Globals.cpp:150). This leads to assert
in Debug and undefined behaviour in Release builds.

The proposed fix generates a name of an unnamed global variable as
__unnamed_<unsigned number>, in a style of similar existing LLVM
implementation (see lib/IR/Mangler.cpp:131). A new class member variable
is added into `SPIRVInstructionSelector` class to keep track of the
number we give to anonymous global values to generate the same name
every time when this is needed.

The patch adds a new LIT test with the smallest implementation of
reproducer ll code.