[ELF] Support . and $ in symbol names in expressions

GNU ld supports `.` and `$` in symbol names while LLD doesn't support them in
`readPrimary` expressions. Using `.` can result in such an error:

```
https://github.com/ClangBuiltLinux/linux/issues/1318
ld.lld: error: ./arch/powerpc/kernel/vmlinux.lds:255: malformed number: .TOC.
>>>   __toc_ptr = (DEFINED (.TOC.) ? .TOC. : ADDR (.got)) + 0x8000;
```

Allow `.` (ppc64 special symbol `.TOC.`) and `$` (RISC-V special symbol `__global_pointer$`).

Change `diag[3-5].test` to use an invalid character `^`.

Note: GNU ld allows `~` in non-leading positions of a symbol name.  `~`
is not used in practice, conflicts with the unary operator, and can
cause some parsing difficulty, so this patch does not add it.

Differential Revision: https://reviews.llvm.org/D98306

lld/ELF/ScriptParser.cpp

@@ -1234,6 +1234,13 @@ static void checkIfExists(OutputSection *cmd, StringRef location) {
     error(location + ": undefined section " + cmd->name);
 }
 
+static bool isValidSymbolName(StringRef s) {
+  auto valid = [](char c) {
+    return isAlnum(c) || c == '$' || c == '.' || c == '_';
+  };
+  return !s.empty() && !isDigit(s[0]) && llvm::all_of(s, valid);
+}
+
 Expr ScriptParser::readPrimary() {
   if (peek() == "(")
     return readParenExpr();
@@ -1408,7 +1415,7 @@ Expr ScriptParser::readPrimary() {
     return [=] { return *val; };
 
   // Tok is a symbol name.
-  if (!isValidCIdentifier(tok))
+  if (!isValidSymbolName(tok))
     setError("malformed number: " + tok);
   script->referencedSymbols.push_back(tok);
   return [=] { return script->getSymbolValue(tok, location); };