Use the real compiler for compdb

Some tools (eg ccls) require that the compiler in argv[0] of the
compdb be an actual compiler binary. Until now we had simply filled
that slot with /bin/false. This change fills it with the current
default clang binary instead.

Test: make -j50 SOONG_GEN_COMPDB=1 SOONG_GEN_COMPDB_DEBUG=1 SOONG_LINK_COMPDB_TO=$ANDROID_BUILD_TOP nothing
Test: Use ccls
Change-Id: I920e8d3113e398b629228070a904dbf1535856e0