[WebAssembly] Fixed relocation errors having no location.

[aardappel]

Summary

Fixes: https://bugs.llvm.org/show_bug.cgi?id=42441

Used to print:

<unknown>:0: error: Cannot represent a difference across sections

(the location was null).

Now prints:

err.s:20:3: error: Cannot represent a difference across sections

i32.const foo-bar
^

Note: I looked at adding a test for this, but I don't think it is
worth it. We're not testing error formatting in the Wasm backend :)

[jgravelle-google]

lgtm

lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp

bool ParseDirective(AsmToken DirectiveID) override {
     return true; // We didn't process this directive.
   }
 
   bool MatchAndEmitInstruction(SMLoc IDLoc, unsigned & /*Opcode*/,
                                OperandVector &Operands, MCStreamer &Out,
                                uint64_t &ErrorInfo,
                                bool MatchingInlineAsm) override {
     MCInst Inst;
+    Inst.setLoc(IDLoc);
     unsigned MatchResult =
         MatchInstructionImpl(Operands, Inst, ErrorInfo, MatchingInlineAsm);
     switch (MatchResult) {
     case Match_Success: {
       if (CurrentState == FunctionStart) {
         // This is the first instruction in a function, but we haven't seen
         // a .local directive yet. The streamer requires locals to be encoded
         // as a prelude to the instructions, so emit an empty list of locals