Update the discriminator assignment algorithm

* If a scope has already been assigned a discriminator, do not reassign a nested discriminator for it.
* If the file and line both match, even if the column does not match, we should assign a new discriminator for the stmt.

original code:
; #1 int foo(int i) {
; #2 if (i == 3 || i == 5) return 100; else return 99;
; #3 }

; i == 3: discriminator 0
; i == 5: discriminator 2
; return 100: discriminator 1
; return 99: discriminator 3

llvm-svn: 251680