[this re-applies c0176916a4824812d25a5a22c4ff7c95857b0cd6
with the correct commit message and phabricator link]
This addresses point 1 of PR44213.
https://bugs.llvm.org/show_bug.cgi?id=44213
The DW_AT_LLVM_sysroot attribute is used for Clang module debug info,
to allow LLDB to import a Clang module from source. Currently it is
part of each DW_TAG_module, however, it is the same for all modules in
a compile unit. It is more efficient and less ambiguous to store it
once in the DW_TAG_compile_unit.
This should have no effect on DWARF consumers other than LLDB.
Differential Revision: https://reviews.llvm.org/D71732
3363ed7b0a