when emitting diagnostics about some character in a numeric

when emitting diagnostics about some character in a numeric
literal, indicate what character the error is about or where
it would be.  For example:

pr2241.c:1:17: error: exponent has no digits
float f[] = { 1e  , 1ee0 };
                ^

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@49996 91177308-0d34-0410-b5e6-96231b3b80d8