[clang] Enable clang build with LLVM_BUILD_INSTRUMENTED without setting LLVM_PROFTDATA

At the moment if LLVM_BUILD_INSTRUMENTED is set to True 
one has to set LLVM_PROFTDATA even if it's not really used 
(because of message(FATAL_ERROR ...)). 
Building the instrumented version of Clang can be useful even if 
one doesn't plan to build the target generate-profdata
(currently that target would only compile utils/perf-training/cxx/hello_world.cpp).
For example, one can run the instrumented version of Clang 
via a separate build system against a different codebase, 
collect/analyze the profiles and merge them by llvm-profdata later.

Differential revision: https://reviews.llvm.org/D38859

llvm-svn: 315665