Revert "Ninja: Use full path for all source files"

This reverts commit v3.7.0-rc1~275^2 (Ninja: Use full path for all
source files, 2016-08-05).  Unfortunately using absolute paths can
cause incorrect rebuilds due to ninja limitations.  The ninja
manual [1] explains:

> ... using absolute paths, your depfile may result in a mixture of
> relative and absolute paths. Paths used by other build rules need
> to match exactly.

Passing an absolute path to a source file to the compiler while using a
relative path in the ninja build manifest can cause such mixture and
lead to incorrect rebuilds.  Simply revert the change for now.

Note that there was a follow-up to the original change in commit
v3.7.0-rc2~10^2 (Ninja: Fix RC language depfile generation with
cmcldeps, 2016-10-13).  We don't need to revert that because that
change made the relevant code cleverly adapt to whatever variable
we use to reference the source file.

[1] https://ninja-build.org/manual.html#_deps

Fixes: #16675
Issue: #13894