Fix a bug when scan-build is used in a cross-compilation environment with

the --use-cc option.

Instead, we will search in the PATH
For example:
 scan-build --use-cc=arm-none-eabi-gcc -o out make -e

Initially reported as a Debian Bug:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=748777

llvm-svn: 215229