[Test] Trim unnecessary .c and .cpp from config.suffix in lit.local.cfg

Tested by comparing make check VERBOSE=1 before and after to make sure
no tests are missed.  (VERBOSE=1 prints the list of tests.)

Only one test :( remains where .cpp is required:

tools/llvm-cov/range_based_for.cpp:// RUN: llvm-cov range_based_for.cpp | FileCheck %s --check-prefix=STDOUT

The topic was discussed in this thread:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140428/214905.html