[ms-inline asm] Add error check for `getAsInteger`

[pengfei]

Summary

.Imm can get lexed as a real, but a real doesn't equal to .Imm, e.g.,
2.5 or .123e+8. We should report error for it rather than silently ignore.

[skan]

Add a test?

[pengfei]

In D151652#4379594, @skan wrote:

Add a test?

I don't think it's necessary to check each negative inputs, especially we don't have actual problem with this. This was just found by static analyze tool.
We even don't have a test when the functionality was introduced rG5dcb4664f276

[skan]

LGTM

llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp

bool X86AsmParser::ParseIntelDotOperator(IntelExprStateMachine &SM,
   StringRef DotDispStr = Tok.getString();
   if (DotDispStr.startswith("."))
     DotDispStr = DotDispStr.drop_front(1);
   StringRef TrailingDot;
 
   // .Imm gets lexed as a real.
   if (Tok.is(AsmToken::Real)) {
     APInt DotDisp;
-    DotDispStr.getAsInteger(10, DotDisp);
+    if (DotDispStr.getAsInteger(10, DotDisp))
+      return Error(Tok.getLoc(), "Unexpected offset");
     Info.Offset = DotDisp.getZExtValue();
   } else if ((isParsingMSInlineAsm() || getParser().isParsingMasm()) &&
              Tok.is(AsmToken::Identifier)) {
     if (DotDispStr.endswith(".")) {
       TrailingDot = DotDispStr.substr(DotDispStr.size() - 1);
       DotDispStr = DotDispStr.drop_back(1);
     }
     const std::pair<StringRef, StringRef> BaseMember = DotDispStr.split('.');