[MLIR][LLVM] Add distinct identifier to DICompileUnit attribute

This commit adds a distinct attribute parameter to the DICompileUnit to
enable the modeling of distinctness. LLVM requires DICompileUnits to be
distinct and there are cases where one gets two equivalent compilation
units but LLVM still requires differentiates them. We observed such
cases for combinations of LTO and inline functions.

This patch also changes the DIScopeForLLVMFuncOp pass to a module pass,
to ensure that only one distinct DICompileUnit is created, instead of
one for each function.