When AST merging for record declarations fails, warn about the

incompatibility and show where the structural differences are. For
example:

struct1.c:36:8: warning: type 'struct S7' has incompatible definitions
in different translation units
struct S7 { int i : 8; unsigned j : 8; } x7;
       ^
struct1.c:36:33: note: bit-field 'j' with type 'unsigned int' and length 8 here
struct S7 { int i : 8; unsigned j : 8; } x7;
                                ^
struct2.c:33:33: note: bit-field 'j' with type 'unsigned int' and length 16 here
struct S7 { int i : 8; unsigned j : 16; } x7;
                                ^

There are a few changes to make this work:
  - ASTImporter now has only a single Diagnostic object, not multiple
  diagnostic objects. Otherwise, having a warning/error printed via
  one Diagnostic and its note printed on the other Diagnostic could
  cause the note to be suppressed.
  - Implemented import functionality for IntegerLiteral (along with
  general support for statements and expressions)

llvm-svn: 95900