Add a new warning to -Wliteral-conversion to catch cases where a string literal

Add a new warning to -Wliteral-conversion to catch cases where a string literal
is cast to a boolean.  An exception has been made for string literals in
logical expressions to allow the common case of use in assert statements.

bool x;
x = "hi";  // Warn here
void foo(bool x);
foo("hi");  // Warn here
assert(0 && "error");
assert("error);  // Warn here

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@140405 91177308-0d34-0410-b5e6-96231b3b80d8