Improve diagnostics when we fail to convert from a source type to a

Improve diagnostics when we fail to convert from a source type to a
destination type for initialization, assignment, parameter-passing,
etc. The main issue fixed here is that we used rather confusing
wording for diagnostics such as

t.c:2:9: warning: initializing 'char const [2]' discards qualifiers,
      expected 'char *' [-pedantic]
  char *name = __func__;
        ^      ~~~~~~~~

We're not initializing a 'char const [2]', we're initializing a 'char
*' with an expression of type 'char const [2]'. Similar problems
existed for other diagnostics in this area, so I've normalized them all
with more precise descriptive text to say what we're
initializing/converting/assigning/etc. from and to. The warning for
the code above is now:

t.c:2:9: warning: initializing 'char *' from an expression of type
      'char const [2]' discards qualifiers [-pedantic]
  char *name = __func__;
        ^      ~~~~~~~~

Fixes <rdar://problem/7447179>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@100832 91177308-0d34-0410-b5e6-96231b3b80d8