Move the body of GCCInstallationDetector ctor into an init() function
and call it from its only user. The linux toolchain. This saves quite
a lot of directory searching on other platforms.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196590 91177308-0d34-0410-b5e6-96231b3b80d8