Re-commit "Use StringRef in Support/Darf APIs (NFC)"

This reverts commit r283285 and re-commit r283275 with
a fix for format("%s", Str); where Str is a StringRef.

llvm-svn: 283298
diff --git a/llvm/unittests/Support/DwarfTest.cpp b/llvm/unittests/Support/DwarfTest.cpp
index 74fcc98..148ea27 100644
--- a/llvm/unittests/Support/DwarfTest.cpp
+++ b/llvm/unittests/Support/DwarfTest.cpp
@@ -17,13 +17,13 @@
 
 TEST(DwarfTest, TagStringOnInvalid) {
   // This is invalid, so it shouldn't be stringified.
-  EXPECT_EQ(nullptr, TagString(DW_TAG_invalid));
+  EXPECT_EQ(StringRef(), TagString(DW_TAG_invalid));
 
   // These aren't really tags: they describe ranges within tags.  They
   // shouldn't be stringified either.
-  EXPECT_EQ(nullptr, TagString(DW_TAG_lo_user));
-  EXPECT_EQ(nullptr, TagString(DW_TAG_hi_user));
-  EXPECT_EQ(nullptr, TagString(DW_TAG_user_base));
+  EXPECT_EQ(StringRef(), TagString(DW_TAG_lo_user));
+  EXPECT_EQ(StringRef(), TagString(DW_TAG_hi_user));
+  EXPECT_EQ(StringRef(), TagString(DW_TAG_user_base));
 }
 
 TEST(DwarfTest, getTag) {
@@ -58,12 +58,12 @@
 
 TEST(DwarfTest, LanguageStringOnInvalid) {
   // This is invalid, so it shouldn't be stringified.
-  EXPECT_EQ(nullptr, LanguageString(0));
+  EXPECT_EQ(StringRef(), LanguageString(0));
 
   // These aren't really tags: they describe ranges within tags.  They
   // shouldn't be stringified either.
-  EXPECT_EQ(nullptr, LanguageString(DW_LANG_lo_user));
-  EXPECT_EQ(nullptr, LanguageString(DW_LANG_hi_user));
+  EXPECT_EQ(StringRef(), LanguageString(DW_LANG_lo_user));
+  EXPECT_EQ(StringRef(), LanguageString(DW_LANG_hi_user));
 }
 
 TEST(DwarfTest, getLanguage) {
@@ -85,12 +85,12 @@
 
 TEST(DwarfTest, AttributeEncodingStringOnInvalid) {
   // This is invalid, so it shouldn't be stringified.
-  EXPECT_EQ(nullptr, AttributeEncodingString(0));
+  EXPECT_EQ(StringRef(), AttributeEncodingString(0));
 
   // These aren't really tags: they describe ranges within tags.  They
   // shouldn't be stringified either.
-  EXPECT_EQ(nullptr, AttributeEncodingString(DW_ATE_lo_user));
-  EXPECT_EQ(nullptr, AttributeEncodingString(DW_ATE_hi_user));
+  EXPECT_EQ(StringRef(), AttributeEncodingString(DW_ATE_lo_user));
+  EXPECT_EQ(StringRef(), AttributeEncodingString(DW_ATE_hi_user));
 }
 
 TEST(DwarfTest, getAttributeEncoding) {
@@ -122,8 +122,8 @@
             VirtualityString(DW_VIRTUALITY_max));
 
   // Invalid numbers shouldn't be stringified.
-  EXPECT_EQ(nullptr, VirtualityString(DW_VIRTUALITY_max + 1));
-  EXPECT_EQ(nullptr, VirtualityString(DW_VIRTUALITY_max + 77));
+  EXPECT_EQ(StringRef(), VirtualityString(DW_VIRTUALITY_max + 1));
+  EXPECT_EQ(StringRef(), VirtualityString(DW_VIRTUALITY_max + 77));
 }
 
 TEST(DwarfTest, getVirtuality) {