Fix PR22750: non-determinism causes assertion failure in DWARF generation

The cause of the issue is the interaction of two factors:
1) When generating a DW_TAG_imported_declaration DIE which imports another 
   imported declaration, the code in AsmPrinter/DwarfCompileUnit.cpp 
   asserts that the second imported declaration must already have a DIE.
2) There is a non-determinism in the order in which imported declarations
   within the same scope are processed.
Because of the non-determinism (2), it is possible that an imported 
declaration is processed before another one it depends on, breaking the 
assumption in (1).

The source of the non-determinism is that the imported declaration 
DIDescriptors are sorted by scope in DwarfDebug::beginModule(); however that
sort is not a stable_sort, therefore the order of the declarations within 
the same scope is not preserved. The attached patch changes the std::sort to
a std::stable_sort and it fixes the problem.

Test omitted due to it being non-deterministic and depending on the
implementation of std::sort.

llvm-svn: 231100