Update ccc driver

 - Use CCC_ECHO to control echoing behavior (default is on, set to ''
   or unset to disable)
 - Get 'clang','llc','as','cc','ld' executables from environment. 'cc'
   and 'ld' are fetched through CCC_CC and CCC_LD respectively -- to
   support make CC=ccc -- the others are through their upcased
   versions.
 - Add CCC_NATIVE mode.
   o This uses llc and as to generate native object files; allowing
     more drop-in replacement of gcc.
   o Disabled by default, but should eventually be default.
   o Allow --emit-llvm in CCC_NATIVE mode to override. Essentially
     this makes ccc more like a drop in replacement for llvm-gcc.
 - Prevent explicit -x <language> options from annoying clang.

llvm-svn: 55260