llvm-cov: Split GCOVFile's read into GCNO and GCDA.

This splits the file-scope read() function into readGCNO() and
readGCDA(). Also broke file format read into functions that first read
the file type, then check the version.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196353 91177308-0d34-0410-b5e6-96231b3b80d8