[sancov] Accommodate sancov and coverage report server for use under Windows

Summary:
This patch makes the following changes to SanCov and its complementary Python script in order to resolve issues pertaining to non-UNIX file paths in JSON symbolization information:
* Convert all paths to use forward slash.
* Update `coverage-report-server.py` to correctly handle paths to sources which contain spaces.
* Remove Linux platform restriction for all SanCov unit tests. All SanCov tests passed when ran on my local Windows machine.

Patch by Douglas Gliner.

Reviewers: kcc, filcab, phosek, morehouse, vitalybuka, metzman

Reviewed By: vitalybuka

Subscribers: vsk, Dor1s, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D51018