fix a couple of problems with section attributes:

1. Passing something that isn't a string used to cause:
   "argument to annotate attribute was not a string literal"
  make it say "section attribute" instead.

2. Fix the location of the above message to point to the
   bad argument instead of the section token.

3. Implement rdar://4341926, by diagnosing invalid section
   specifiers in the frontend rather than letting them slip all
   the way to the assembler (a QoI win).

An example of #3 is that we used to produce something like this:

/var/folders/n7/n7Yno9ihEm894640nJdSQU+++TI/-Tmp-//ccFPFGtT.s:2:Expected comma after segment-name
/var/folders/n7/n7Yno9ihEm894640nJdSQU+++TI/-Tmp-//ccFPFGtT.s:2:Rest of line ignored. 1st junk character valued 46 (.).

Daniel improved clang to use llvm_report_error, so now we got:

$ clang t.c -c
fatal error: error in backend: Global variable 'x' has an invalid section specifier 'sadf': mach-o section specifier
      requires a segment and section separated by a comma.

with no loc info.  Now we get:

$ clang t.c -fsyntax-only
t.c:4:30: error: argument to 'section' attribute is not valid for this target: mach-o section specifier requires a segment
      and section separated by a comma
int x __attribute__((section("sadf")));
                             ^

which is nice :)



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@78586 91177308-0d34-0410-b5e6-96231b3b80d8