When forming the squigly underline for a diagnostic, make sure to

verify that the source range corresponds to the current file, not
just the current line.  This allows us to emit:

a.c:1:44: error: invalid operands to binary expression ('double' and 'int *')
double a; int *b; void f(void) { int c = a +
                                         ~ ^

instead of:

a.c:1:44: error: invalid operands to binary expression ('double' and 'int *')
double a; int *b; void f(void) { int c = a +
~                                        ~ ^

for PR1906 (note the leading ~).

Thanks to Neil for noticing this.

llvm-svn: 45901