Rename DW_AT_LLVM_isysroot to DW_AT_LLVM_sysroot

This is a purely cosmetic change that is NFC in terms of the binary
output. I bugs me that I called the attribute DW_AT_LLVM_isysroot
since the "i" is an artifact of GCC command line option syntax
(-isysroot is in the category of -i options) and doesn't carry any
useful information otherwise.

This attribute only appears in Clang module debug info.

Differential Revision: https://reviews.llvm.org/D71722

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();