[WebAssembly] Fix error location for parsed symbol/label operands

The previous code didn't take account for the fact that parseExpression
my lex additional tokens - because of this, it's necessary to record the
location of the current token ahead of the call. This patch additionally
makes use of the fact parseExpression will set its End parameter to the
end of the expression.

Although this fix could be added independently of D122127, I've opted to
make it a child patch in order to ensure the change has some test
coverage.

Differential Revision: https://reviews.llvm.org/D122128