Give explicit sections for string constants used in NSStrings.

Without them they can be merged with non unnamed_addr constants during LTO.
The resulting constant is not unnamed_addr and goes in a different section,
which causes ld64 to crash.

A testcase that would crash before:

* file1.mm:
void g(id notification) {
  [notification valueForKey:@"name"];
}

* file2.cpp:
extern const char js_name_str[] = "name";

* file3.cpp
extern bool JS_GetProperty(const char *name);
extern const char js_name_str[];
bool js_ReportUncaughtException() { JS_GetProperty(js_name_str); }

run

clang file1.mm  -o file1.o -c -w -emit-llvm
clang file2.cpp -o file2.o -c -w -emit-llvm
clang file3.cpp -o file3.o -c -w

ld -dylib -o XUL file1.o file2.o file3.o -undefined dynamic_lookup.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199688 91177308-0d34-0410-b5e6-96231b3b80d8