Introduce llvm-cov.

Introduce llvm-cov.

Add llvm-cov skeleton. It has initial support to read coverage info generated by GCOVProfiling.cpp.
Today, you can do
prompt> clang a.c -ftest-coverage -fprofile-arcs -o a
prompt> ./a
prompt> llvm-cov -gcno a.gcno -gcda a.gcda
a.c
 :	#include "a.h"
 :
 :	int main() {
 :		int i = 0;
 :		if (i) {
1:			int j = 0;
1:			j = 1;
1:		} else {
 :			int k = 1;
 :			k = 2;
 :		}
1:		return 0;
 :	}
 :
 :

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@140712 91177308-0d34-0410-b5e6-96231b3b80d8