Recommit [SampleFDO] Expose an interface to return the size of a section

or the size of the profile for profile in ExtBinary format.

Fix a test failure on Mac.

[SampleFDO] Expose an interface to return the size of a section or the
size of the profile for profile in ExtBinary format.

Sometimes we want to limit the size of the profile by stripping some functions
with low sample count or by stripping some function names with small text size
from profile symbol list. That requires the profile reader to have the
interfaces returning the size of a section or the size of total profile. The
patch add those interfaces.

At the same time, add some dump facility to show the size of each section.

Differential revision: https://reviews.llvm.org/D67726

llvm-svn: 372478