I (re)discovered that dsymutil was instantiating two BinaryHolders: one
for parsing the debug map and one for linking. That really defeats the
purpose of the BinaryHolder as it serves as a cache. Fix the issue and
remove an old FIXME.

Remove excess layer of indirection.

When the MachODebugMapParser encounters an object file that cannot be
found on disk, it currently leaves the parser in an incoherent state,
resulting in spurious warnings that can in turn slow down dsymutil.

This fixes #78411.

rdar://117515153

The current warning emitted by dsymutil when it can't find a symbol in
an object file is worded rather poorly:

```
could not find object file symbol for symbol _foo
```

It's also lacking information that makes the warning actionable, such as
the object file it's looking at. This patch rewords the warning and adds
the object file path to the warning:

```
could not find symbol '_foo' in object file 'test.o'
```

rdar://119621065

This patch replaces uses of StringRef::{starts,ends}with with
StringRef::{starts,ends}_with for consistency with
std::{string,string_view}::{starts,ends}_with in C++20.

I'm planning to deprecate and eventually remove
StringRef::{starts,ends}with.

Reland https://reviews.llvm.org/D158124
Fixed `-fpermissive` error reported by gcc only.

This reverts commit 122c89b271af30b86536cad7bac64ea9c56615ed. Change does not build, with errors such as:

In file included from ../llvm-project/llvm/tools/dsymutil/DebugMap.h:24,
                 from ../llvm-project/llvm/tools/dsymutil/DwarfLinkerForBinary.h:13,
                 from ../llvm-project/llvm/tools/dsymutil/DwarfLinkerForBinary.cpp:9:
../llvm-project/llvm/tools/dsymutil/RelocationMap.h:60:17: error: declaration of ‘llvm::dsymutil::SymbolMapping llvm::dsymutil::ValidReloc::SymbolMapping’ changes meaning of ‘SymbolMapping’ [-fpermissive]
   60 |   SymbolMapping SymbolMapping;
      |                 ^~~~~~~~~~~~~
../llvm-project/llvm/tools/dsymutil/RelocationMap.h:36:8: note: ‘SymbolMapping’ declared here as ‘struct llvm::dsymutil::SymbolMapping’
   36 | struct SymbolMapping {
      |        ^~~~~~~~~~~~~
In file included from ../llvm-project/llvm/tools/dsymutil/DwarfLinkerForBinary.h:13,
                 from ../llvm-project/llvm/tools/dsymutil/DwarfLinkerForBinary.cpp:9:
../llvm-project/llvm/tools/dsymutil/DebugMap.h:198:32: error: declaration of ‘std::optional<llvm::dsymutil::RelocationMap> llvm::dsymutil::DebugMapObject::RelocationMap’ changes meaning of ‘RelocationMap’ [-fpermissive]
  198 |   std::optional<RelocationMap> RelocationMap;
      |                                ^~~~~~~~~~~~~
In file included from ../llvm-project/llvm/tools/dsymutil/DebugMap.h:24,
                 from ../llvm-project/llvm/tools/dsymutil/DwarfLinkerForBinary.h:13,
                 from ../llvm-project/llvm/tools/dsymutil/DwarfLinkerForBinary.cpp:9:
../llvm-project/llvm/tools/dsymutil/RelocationMap.h:76:7: note: ‘RelocationMap’ declared here as ‘class llvm::dsymutil::RelocationMap’
   76 | class RelocationMap {
      |       ^~~~~~~~~~~~~

This adds support in dsymutil for mergeable libraries [1].

dsymutil reads a new stab emitted by ld, allowing it to operate on
dynamic libraries instead of object files. It also now loads the DWARF
files associated to the libraries, and build the debug map for each
binary from the list of symbols exported by the library. For each Debug
Map Object, there is a new associated Relocation Map which is serialized
from the information retrieved in the original debug_info (or
debug_addr) section of the .o file.

The final DWARF file has multiple compile units, so the offsets
information of the relocations are adjusted relatively to the compile
unit they will end up belonging to, inside the final linked DWARF file.

[1] https://developer.apple.com/documentation/xcode/configuring-your-project-to-use-mergeable-libraries

Differential revision: https://reviews.llvm.org/D158124