[lldb-tests] Add compiler version check in TestFunctionStarts

This test requires compiling its input program without debug
information. To do so, it uses certain Makefile variables that are never
populated with custom libcxx paths (if present). Doing so would not
necessarily be correct: we cannot guarantee that said standard library
has no debug symbols.

As such, we keep using the system libraries but disable the tests in
clang versions that are too old to work with more modern system
libraries, as in the case of the lldb-matrix bot.

Differential Revision: https://reviews.llvm.org/D136178