Driver: Improve diagnostics for failed commands.

 - Not all tools give good error messages, and sometimes the tool can fail w/o
   any error (for example, when signalled).

 - We suppress this message when the failing command is the compiler and it
   failed normally (exit code == 1), under the assumption that it gave a good
   diagnostic.

For example, for a linker failure we now get:
--
ddunbar@lordcrumb:tmp$ clang a.c b.c
ld: duplicate symbol _x in /var/folders/cl/clrOX6SaG+moCeRKEI4PtU+++TI/-Tmp-/cc-bXYITq.o and /var/folders/cl/clrOX6SaG+moCeRKEI4PtU+++TI/-Tmp-/cc-6uK4jD.o
clang: error: linker command failed with exit code 1 (use -v to see invocation)
--

For a compiler crash we get:
--
ddunbar@lordcrumb:tmp$ clang t.i
Assertion failed: (CGT.getTargetData().getTypeAllocSizeInBits(STy) == RL.getSize()), function layoutStructFields, file CodeGenTypes.cpp, line 573.
0   clang-cc          0x0000000100f1f1f1 PrintStackTrace(void*) + 38
... stack trace and virtual stack trace follow ...
clang: error: compiler command failed due to signal 6 (use -v to see invocation)
--

But for a regular compilation failure we get the usual:
--
ddunbar@lordcrumb:tmp$ clang c.c
c.c:1:6: error: invalid token after top level declarator
int x
     ^
1 diagnostic generated.
--

 - No test case, not amenable to non-executable testing. :/

 - <rdar://problem/6945613>

llvm-svn: 74629