[LLVM-COV]Replace tabs to the space indentations in the HTML coverage report.

When using orbis-llvm-cov.exe to generate the HTML report, the HTML report 
can look quite different to the source file if it includes tabs.The default
tab size is 2 spaces instead of 8 spaces. A command line switch is
be added to set the tab size.

Differential Revision: https://reviews.llvm.org/D23087

llvm-svn: 277715