Directly build the profile file name while merging

Bug: http://b/37574207

build.pgo_profile_file() actually returns a full path, which is not what
we want when invoking llvm-profdata to create the profile file.
Instead, just build the profile file name directly when invoking
llvm-profdata.

In light of this, sink the os.path.exists() check back to
pgo_profile_file().

Test: test_compiler.py with --generate-clang-profile while
toolchain/llvm/android/profiles is absent.

Change-Id: Ic8c332654d2c0d7e03217d7d1982c3a5fd9fc937