lang: Import compiler-rt version 6.0.1.

The compiler-rt project consists of:

- builtins - a simple library that provides an implementation of the low-level
  target-specific hooks required by code generation and other runtime
  components. For example, when compiling for a 32-bit target, converting a
  double to a 64-bit unsigned integer is compiling into a runtime call to the
  "__fixunsdfdi" function. The builtins library provides optimized
  implementations of this and other low-level routines, either in
  target-independent C form, or as a heavily-optimized assembly.

- sanitizer runtimes - runtime libraries that are required to run the code
  with sanitizer instrumentation. This includes runtimes for:
  *    AddressSanitizer
  *    ThreadSanitizer
  *    UndefinedBehaviorSanitizer
  *    MemorySanitizer
  *    LeakSanitizer
  *    DataFlowSanitizer

- profile - library which is used to collect coverage information.

- BlocksRuntime - a target-independent implementation of Apple "Blocks"
  runtime interfaces.

(jperkin)