Move the sysroot attribute from DIModule to DICompileUnit

[this re-applies c0176916a4
 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

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -1662,6 +1662,8 @@ void ModuleBitcodeWriter::writeDICompileUnit(const DICompileUnit *N,
   Record.push_back(N->getSplitDebugInlining());
   Record.push_back(N->getDebugInfoForProfiling());
   Record.push_back((unsigned)N->getNameTableKind());
+  Record.push_back(N->getRangesBaseAddress());
+  Record.push_back(VE.getMetadataOrNullID(N->getRawSysRoot()));
 
   Stream.EmitRecord(bitc::METADATA_COMPILE_UNIT, Record, Abbrev);
   Record.clear();