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

[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.

llvm-svn: 372439