Revert "Remove env-var-allowlist"

Revert "Add -fdebug-compilation-dir option"

Revert submission 1461902-debug-compilation-dir

Reason for revert: "-Xclang" isn't being uniformly respected everywhere. For example, in ".S" compilations, when I pass `-Xclang -fdebug-compilation-dir=.", the assembler seems to be ignoring it and then inserting the `pwd` into the command, however when I pass "-fdebug-compilation-dir=.", it strips out the path to the current working directory.
This indicates that we need to update re-client's input processor so that we can pass -fdebug-compilation-dir=. without "-Xclang" and then remove `PWD` setting.
For now, I'll update this patch to pass both "-fdebug-compilation-dir=." and `PWD` and when RBE side fix is done, I'll remove `PWD` in a separate CL.

Reverted Changes:
Ib0f271e55:Add -fdebug-compilation-dir option
Ifa0592af5:Remove env-var-allowlist

Change-Id: I7c690b3e00d37dbcc8fbaa66dda49f39032be3ab