[SampleFDO] Allow forward compatibility when adding a new section for extbinary

format.

Currently when we add a new section in the profile format and generate a profile
containing the new section, older compiler which reads the new profile will
issue an error. The forward incompatibility can cause unnecessary churn when
extending the profile. This patch removes the incompatibility when adding a new
section for extbinary format.

Differential Revision: https://reviews.llvm.org/D109398