[BOLT] Report per-section hotness in bolt-heatmap.

This patch adds a new feature to bolt heatmap to print the hotness of each section in terms of the percentage of samples within that section.

Sample output generated for the clang binary:

Section Name, Begin Address, End Address, Percentage Hotness
.text, 0x1a7b9b0, 0x20a2cc0, 1.4709
.init, 0x20a2cc0, 0x20a2ce1, 0.0001
.fini, 0x20a2ce4, 0x20a2cf2, 0.0000
.text.unlikely, 0x20a2d00, 0x431990c, 0.3061
.text.hot, 0x4319910, 0x4bc6927, 97.2197
.text.startup, 0x4bc6930, 0x4c10c89, 0.0058
.plt, 0x4c10c90, 0x4c12010, 0.9974

Reviewed By: rafauler

Differential Revision: https://reviews.llvm.org/D124412