When clang is host compiler, autofdo does not build with compiler-rt as default rtlib.

https://uberchromegw.corp.google.com/i/chromiumos.tryserver/builders/chromiumos-sdk/builds/2305

autofdo-0.15-r3: rm -f libglog.a
autofdo-0.15-r3: ar cru libglog.a glog/src/libglog_a-logging.o glog/src/libglog_a-raw_logging.o glog/src/libglog_a-vlog_is_on.o glog/src/libglog_a-utilities.o glog/src/libglog_a-demangle.o glog/src/libglog_a-symbolize.o glog/src/libglog_a-signalhandler.o 
autofdo-0.15-r3: x86_64-pc-linux-gnu-ranlib libglog.a
autofdo-0.15-r3: llvm_profile_writer.cc:101:18: error: no member named 'setName' in
autofdo-0.15-r3:       'llvm::StringMap<llvm::sampleprof::FunctionSamples,
autofdo-0.15-r3:       llvm::MallocAllocator>'
autofdo-0.15-r3:   callee_profile.setName(GetNameRef(callsite.second));
autofdo-0.15-r3:   ~~~~~~~~~~~~~~ ^
autofdo-0.15-r3: llvm_profile_writer.cc:102:17: error: no matching member function for call to
autofdo-0.15-r3:       'push_back'
autofdo-0.15-r3:   inline_stack_.push_back(&callee_profile);
autofdo-0.15-r3:   ~~~~~~~~~~~~~~^~~~~~~~~
autofdo-0.15-r3: /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/4.9.x/include/g++-v4/bits/stl_vector.h:1038:7: note: 
autofdo-0.15-r3:       candidate function not viable: no known conversion from
autofdo-0.15-r3:       'llvm::StringMap<llvm::sampleprof::FunctionSamples, llvm::MallocAllocator>
autofdo-0.15-r3:       *' to 'const std::vector<llvm::sampleprof::FunctionSamples *,
autofdo-0.15-r3:       std::allocator<llvm::sampleprof::FunctionSamples *> >::value_type' (aka
autofdo-0.15-r3:       'llvm::sampleprof::FunctionSamples *const') for 1st argument
autofdo-0.15-r3:       push_back(const value_type& __x)
autofdo-0.15-r3:       ^
autofdo-0.15-r3: /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/4.9.x/include/g++-v4/bits/stl_vector.h:1057:7: note: 
autofdo-0.15-r3:       candidate function not viable: no known conversion from
autofdo-0.15-r3:       'llvm::StringMap<llvm::sampleprof::FunctionSamples, llvm::MallocAllocator>
autofdo-0.15-r3:       *' to 'std::vector<llvm::sampleprof::FunctionSamples *,
autofdo-0.15-r3:       std::allocator<llvm::sampleprof::FunctionSamples *> >::value_type' (aka
autofdo-0.15-r3:       'llvm::sampleprof::FunctionSamples *') for 1st argument
autofdo-0.15-r3:       push_back(value_type&& __x)
autofdo-0.15-r3:       ^
autofdo-0.15-r3: 2 errors generated.
autofdo-0.15-r3: make[1]: *** [Makefile:1059: create_llvm_prof-llvm_profile_writer.o] Error 1