Alternative (to r216344) fix of gcc -Wpedantic.

As suggested by David Blaikie, this may be easier to read.

The original warning was:

../tools/llvm-cov/llvm-cov.cpp:53:49: error: ISO C++ forbids zero-size array 'argv' [-Werror=pedantic]
       std::string Invocation(std::string(argv[0]) + " " + argv[1]);

It seems to be the case that GCC's warning gets confused and thinks
'argv' is a declaration here. GCC bugzilla issue #61259.