Use discriminator information in sample profiles.

Summary:
When the sample profiles include discriminator information,
use the discriminator values to distinguish instruction weights
in different basic blocks.

This modifies the BodySamples mapping to map <line, discriminator> pairs
to weights. Instructions on the same line but different blocks, will
use different discriminator values. This, in turn, means that the blocks
may have different weights.

Other changes in this patch:

- Add tests for positive values of line offset, discriminator and samples.
- Change data types from uint32_t to unsigned and int and do additional
  validation.

Reviewers: chandlerc

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2857