[ELF] Add internal InputFile (#78944)

Based on https://reviews.llvm.org/D45375 . Introduce a new InputFile
kind `InternalKind`, use it for

* `ctx.internalFile`: for linker-defined symbols and some synthesized
`Undefined`
* `createInternalFile`: for symbol assignments and --defsym

I picked "internal" instead of "synthetic" to avoid confusion with
SyntheticSection.

Currently a symbol's file is one of: nullptr, ObjKind, SharedKind,
BitcodeKind, BinaryKind. Now it's non-null (I plan to add an
`assert(file)` to Symbol::Symbol and change `toString(const InputFile
*)`
separately).

Debugging and error reporting gets improved. The immediate user-facing
difference is more descriptive "File" column in the --cref output. This
patch may unlock further simplification.

Currently each symbol assignment gets its own
`createInternalFile(cmd->location)`. Two symbol assignments in a linker
script do not share the same file. Making the file the same would be
nice, but would require non trivial code.

1 files changed, 2 insertions, 1 deletions

diff --git a/lld/ELF/ScriptParser.cpp b/lld/ELF/ScriptParser.cpp
index 4fdb8c7..dd69916 100644
--- a/lld/ELF/ScriptParser.cpp
+++ b/lld/ELF/ScriptParser.cpp
@@ -288,7 +288,8 @@ void ScriptParser::readDefsym(StringRef name) {
   Expr e = readExpr();
   if (!atEOF())
     setError("EOF expected, but got " + next());
-  auto *cmd = make<SymbolAssignment>(name, e, 0, getCurrentLocation());
+  auto *cmd = make<SymbolAssignment>(
+      name, e, 0, getCurrentMB().getBufferIdentifier().str());
   script->sectionCommands.push_back(cmd);
 }