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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@280061 91177308-0d34-0410-b5e6-96231b3b80d8