Add -fdebug-compilation-dir option

The main use of this flag is to be to make both the debug info
and the build command line be independent of the compilation dir.
See: https://reviews.llvm.org/D63387 & https://blog.llvm.org/2019/11/deterministic-builds-with-clang-and-lld.html

Removed -fdebug-prefix-map and PWD variable being set in the compile
command line - these were workarounds we originally did to ensure that
the compile output is independent of the working directory and are no
longer needed now that we have -fdebug-compilation-dir.

Test: Ran "m
out/target/product/crosshatch/obj/SHARED_LIBRARIES/libexif.vendor_intermediates/same_vndk_variants.timestamp"
which fails on an RBE hardened stack without this change and passes with
this change.

Bug: b/169266636
Change-Id: Ib0f271e55f6cf0c89a30e65d3a0b25fe3f3a8f48