Revert "Enable continuous coverage: use '%c' specifier in LLVM_PROFILE_FILE"

Bug: http://b/194128476
Bug: http://b/210012154

This reverts commit e59f0f66.

Coverage metrics dropped for ~10 of the 40 modules.  There are also
regressions in mainline when running tests on older platform builds.

Test: presubmit
Change-Id: I50a011f68dcdc25883a68701c51e7e2aabc5a7dc