Go patch committed: Treat S("") as a string constant

[Ian Lance Taylor <iant@golang.org>]

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This Go frontend patch treats S("") as a string constant.  The
compiler neglected to notice that a conversion from a string constant
to a string type was a valid string constant.

I didn't add a test case because this only caused a compiler failure
when compiling without optimization, which is not the normal case, and
is not a case that we test.

This fixes https://go.dev/issue/56113.

Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.  Committed
to mainline.

Ian

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29b0fe393859729215b0db5d28f2faea30c6ec32
diff --git a/gcc/go/gofrontend/MERGE b/gcc/go/gofrontend/MERGE
index 1c2466090f1..5b95b38a541 100644
--- a/gcc/go/gofrontend/MERGE
+++ b/gcc/go/gofrontend/MERGE
@@ -1,4 +1,4 @@
-164f2aeb1deec4c11e55b8bfb152ff7ff4c1dd4c
+6c188108858e3ae8c8ea8e4cc55427d8cf01bbc8
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.
diff --git a/gcc/go/gofrontend/expressions.cc b/gcc/go/gofrontend/expressions.cc
index 247ae1bba34..71838b14629 100644
--- a/gcc/go/gofrontend/expressions.cc
+++ b/gcc/go/gofrontend/expressions.cc
@@ -4092,6 +4092,9 @@ Type_conversion_expression::do_numeric_constant_value(
 bool
 Type_conversion_expression::do_string_constant_value(std::string* val) const
 {
+  if (this->type_->is_string_type() && this->expr_->type()->is_string_type())
+    return this->expr_->string_constant_value(val);
+
   if (this->type_->is_string_type()
       && this->expr_->type()->integer_type() != NULL)
     {