[LLD] [ELF] Add support for linker script unary plus operator

This commit adds support for linker script unary plus ('+') operator.
It is helpful for improving compatibility between LLD and GNU LD.

Author: partaror

lld/ELF/ScriptParser.cpp

@@ -1489,6 +1489,9 @@ Expr ScriptParser::readPrimary() {
     Expr e = readPrimary();
     return [=] { return -e().getValue(); };
   }
+  if (consume("+")) {
+    return readPrimary();
+  }
 
   StringRef tok = next();
   std::string location = getCurrentLocation();

lld/test/ELF/linkerscript/operators.test

@@ -73,6 +73,7 @@ SECTIONS {
   log2ceil100000000 = LOG2CEIL(0x100000000);
   log2ceil100000001 = LOG2CEIL(0x100000001);
   log2ceilmax = LOG2CEIL(0xffffffffffffffff);
+  unaryadd = +3 + ++5;
 }
 
 # CHECK:      0000000000000002 A unary
@@ -126,6 +127,7 @@ SECTIONS {
 # CHECK-NEXT: 0000000000000020 A log2ceil100000000
 # CHECK-NEXT: 0000000000000021 A log2ceil100000001
 # CHECK-NEXT: 0000000000000040 A log2ceilmax
+# CHECK-NEXT: 0000000000000008 A unaryadd
 
 ## Mailformed number error.
 # RUN: echo "SECTIONS { . = 0x12Q41; }" > %t.script