Differentiate between different compound assignment operators (&=, +=...)

It was noticed that before this change, all operators in statements
such as
Foo a, b;
a ^= b;
a &= b;
etc. will be displayed as uses of Foo::operator+=(), which is obviously
wrong. The reason for this was some rather curious code, which just
treated every of these operators as "Token_assign" in the lexer,
and some other code which treated each "Token_assign" as "+=" in the
analyzer. To fix the issue, this had to be sorted out.
- Token_assign doesn't make sense anyways, since assignments are
represented as "=". So, remove the token completely.
- Instead of lexing every compound assignment operator as "Assign",
yield different token types for each.
- In the code which handles the operators, change the function from
using its own list of tokenkind -- tokenText mapping to use the
official one.
Also included is a unit test for the fix.

REVIEW:110638