Frontend: Fix SourceColumnMap assertion failure on non-ascii characters.

Frontend: Fix SourceColumnMap assertion failure on non-ascii characters.

If there are some non-ascii character in the input source code, the
column index might be smallar than the byte index.  This will result
in two possible assertion failures.  This CL fixes the computation of
the column index and byte index.

1. The assertion in startOfNextColumn() and startOfPreviousColumn()
   should not be raised when the byte index is greater than the column
   index since the non-ascii characters may use more than one bytes to
   store a character in a column.

2. The length of the caret line should be equal to the number of columns
   of source line, instead of the length of the source line.  Otherwise,
   the assertion in selectInterestingSourceRegion will be raised because
   the removed columns plus the kept columns are not greater than the max
   column, which means that we should not remove any column at all.

llvm-svn: 225442