try_emplace can default-construct values, so we do not need to do so
on our own.  Plus, try_emplace(Key) is much simpler/shorter than
insert({Key, LongValueType()}).

Remove support for obfuscated bitcode in dsymutil and the DWARF linker.
We no longer support bitcode submissions and the obfuscation support has
been removed from the rest of the compiler.

rdar://123863918

Reverts llvm/llvm-project#85713 Since it is breaking Linux x64 builds.

Remove support for obfuscated bitcode in dsymutil and the DWARF linker.
We no longer support bitcode submissions and the obfuscation support has
been removed from the rest of the compiler.

rdar://123863918

There was a strange and seemingly unncessary empty string optimization
in NonRelocatableStringPool that I had to remove in order to support
empty strings in the line_str string table, without unconditionally
forcing an empty string to be added to every debug_line_str table.

This patch is extraction from the https://reviews.llvm.org/D126883.
It removes DwarfStringPoolEntryRef::isIndexed() and isIndexed bit
since they are not used.

Differential Revision: https://reviews.llvm.org/D126958

Only using that change in StringRef already decreases the number of
preoprocessed lines from 7837621 to 7776151 for LLVMSupport

Perhaps more interestingly, it shows that many files were relying on the
inclusion of StringRef.h to have the declaration from STLExtras.h. This
patch tries hard to patch relevant part of llvm-project impacted by this
hidden dependency removal.

Potential impact:
- "llvm/ADT/StringRef.h" no longer includes <memory>,
  "llvm/ADT/Optional.h" nor "llvm/ADT/STLExtras.h"

Related Discourse thread:
https://llvm.discourse.group/t/include-what-you-use-include-cleanup/5831

Summary:
This patch relands D71271. The problem with D71271 is that it has cyclic dependency:
CodeGen->AsmPrinter->DebugInfoDWARF->CodeGen. To avoid cyclic dependency this patch
puts implementation for DWARFOptimizer into separate library: lib/DWARFLinker.

Thus the difference between this patch and D71271 is in that DWARFOptimizer renamed into
DWARFLinker and it`s files are put into lib/DWARFLinker.

Reviewers: JDevlieghere, friss, dblaikie, aprantl

Reviewed By: JDevlieghere

Subscribers: thegameg, merge_guards_bot, probinson, mgorny, hiraditya, llvm-commits

Tags: #llvm, #debug-info

Differential Revision: https://reviews.llvm.org/D71839

That refactoring moves NonRelocatableStringpool into common CodeGen folder.
So that NonRelocatableStringpool could be used not only inside dsymutil.

Differential Revision: https://reviews.llvm.org/D71068