[llvm-cov] Use the native path in the coverage report.

The coverage reports contain the source or binary file paths. On Windows, 
the file path might contain the seperators of both '/' and '\'. This patch 
uses the native path in the coverage reports. For example, on Windows, 
all '/' are converted to '\'.

Differential Revision: https://reviews.llvm.org/D23922

llvm-svn: 280061