[ELF] - SEGMENT_START's default argument can be an expression

Our implementation supported integer value previously.
ld can use expression,
for example, it is OK to write
 . = SEGMENT_START("foobar", .);

Patch implements that.

llvm-svn: 281831
diff --git a/lld/test/ELF/linkerscript/Inputs/segment-start.script b/lld/test/ELF/linkerscript/Inputs/segment-start.script
index 95933a2..9f1a593 100644
--- a/lld/test/ELF/linkerscript/Inputs/segment-start.script
+++ b/lld/test/ELF/linkerscript/Inputs/segment-start.script
@@ -2,6 +2,6 @@
 {
   PROVIDE (foobar1 = SEGMENT_START("text-segment", 0x8001));
   PROVIDE (foobar2 = SEGMENT_START("data-segment", 0x8002));
-  PROVIDE (foobar3 = SEGMENT_START("bss-segment", 0x8003));
+  PROVIDE (foobar3 = SEGMENT_START("bss-segment", 0x8000 + (4 - 1)));
   PROVIDE (foobar4 = SEGMENT_START("abc-segment", 0x8004));
 }