swift234

lldb test suite: use just-built clang by default

build-script-impl now takes a --lldb-test-cc flag:

* when not specified, it will default to using the
  just-built clang (i.e. in tree with swift) to
  build the C/C++ test suite test inferior programs.

* when specified and equal to "host-toolchain", the
  test suite test inferiors will be built with
  the same compiler chosen as --host-cc.

* when set to anything else, it is assumed to be the
  full path to a C compiler to use for compiling
  LLDB test suite test inferiors.

Note the LLDB test suite only takes the C compiler and
derives the C++ compiler from the C compiler path.  There
is no need to specify the C++ compiler directly.

Addresses the following issue:
https://bugs.swift.org/browse/SR-1207