Default to using in-tree clang for building test executables

Summary:
Using the in-tree clang should be the default test configuration as that
is the one compiler that we can be sure everyone has (better
reproducibility of test results). Also, it should hopefully reduce the
impact of pr35040.

This also reduces the number of settings which control the compiler
used. LLDB_TEST_C_COMPILER is used for C files and
LLDB_TEST_CXX_COMPILER for C++ files. Both of the settings default to
the in-tree clang.

Reviewers: zturner

Subscribers: mgorny, davide, lldb-commits

Differential Revision: https://reviews.llvm.org/D39215

llvm-svn: 316728