Ilya A. Kriveshko authored Feb 22, 2018 and Brad King committed Feb 27, 2018

When using ccache with clang-tidy, ccache needs to wrap compiler
invocation, rather than cmake invocation.  But it needs to do it without
affecting the command line that iwyu-like tools are receiving.

With this fix, if __run_co_compile is used, compile launcher is passed
using the new --launcher option, but if __run_co_compile is not needed,
compiler launcher is prepended to the command line as before.

To better illustrate the change: with this fix if running clang-tidy
with CXX_COMPILER_LAUNCHER set to "/usr/bin/time;-p;ccache" (time -p
added strictly for illustration purposes), the command line changes
from:

    /usr/bin/time -p ccache cmake -E __run_co_compile \
        --tidy=clang-tidy ... -- g++ ...

to:

    cmake -E __run_co_compile \
        --launcher="/usr/bin/time;-p;ccache" \
        --tidy=clang-tidy ... -- g++ ...

This allows the compiler to be run via the launcher, but leaves tidy
(& friends) invocations unaffected.

Fixes: #16493