[llvm][RISCV] Improve error message for invalid extension letters (#90468)

Previously you got:
clang: error: invalid arch name 'rv64v', first letter should be 'e', 'i'
or 'g'

Which to me, unfamiliar with riscv, reads as if I should have used
"[eig]rv64v". Which is not what clang means.

Include the first bit in the error message to make this clearer:
clang: error: invalid arch name 'rv64v', first letter after 'rv64'
should be 'e', 'i' or 'g'

Author: DavidSpickett

clang/test/Driver/riscv-arch.c

@@ -209,7 +209,7 @@
 // RUN: not %clang --target=riscv32-unknown-elf -march=rv32q -### %s \
 // RUN: -fsyntax-only 2>&1 | FileCheck -check-prefix=RV32-LETTER %s
 // RV32-LETTER: error: invalid arch name 'rv32q',
-// RV32-LETTER: first letter should be 'e', 'i' or 'g'
+// RV32-LETTER: first letter after 'rv32' should be 'e', 'i' or 'g'
 
 // RUN: not %clang --target=riscv32-unknown-elf -march=rv32imcq -### %s \
 // RUN: -fsyntax-only 2>&1 | FileCheck -check-prefix=RV32-ORDER %s
@@ -239,12 +239,12 @@
 // RUN: not %clang --target=riscv32-unknown-elf -march=rv32xabc -### %s \
 // RUN: -fsyntax-only 2>&1 | FileCheck -check-prefix=RV32X %s
 // RV32X: error: invalid arch name 'rv32xabc',
-// RV32X: first letter should be 'e', 'i' or 'g'
+// RV32X: first letter after 'rv32' should be 'e', 'i' or 'g'
 
 // RUN: not %clang --target=riscv32-unknown-elf -march=rv32sabc -### %s \
 // RUN: -fsyntax-only 2>&1 | FileCheck -check-prefix=RV32S %s
 // RV32S: error: invalid arch name 'rv32sabc',
-// RV32S: first letter should be 'e', 'i' or 'g'
+// RV32S: first letter after 'rv32' should be 'e', 'i' or 'g'
 
 // RUN: not %clang --target=riscv32-unknown-elf -march=rv32ix -### %s \
 // RUN: -fsyntax-only 2>&1 | FileCheck -check-prefix=RV32X-NAME %s

llvm/lib/TargetParser/RISCVISAInfo.cpp

@@ -639,7 +639,8 @@ RISCVISAInfo::parseArchString(StringRef Arch, bool EnableExperimentalExtension,
   switch (Baseline) {
   default:
     return createStringError(errc::invalid_argument,
-                             "first letter should be 'e', 'i' or 'g'");
+                             "first letter after \'" + Arch.slice(0, 4) +
+                                 "\' should be 'e', 'i' or 'g'");
   case 'e':
   case 'i':
     break;

llvm/unittests/TargetParser/RISCVISAInfoTest.cpp

@@ -120,10 +120,14 @@ TEST(ParseArchString, RejectsInvalidBaseISA) {
     EXPECT_EQ(toString(RISCVISAInfo::parseArchString(Input, true).takeError()),
               "string must begin with rv32{i,e,g} or rv64{i,e,g}");
   }
-  for (StringRef Input : {"rv32j", "rv64k", "rv32_i"}) {
+
+  for (StringRef Input : {"rv32j", "rv32_i"}) {
     EXPECT_EQ(toString(RISCVISAInfo::parseArchString(Input, true).takeError()),
-              "first letter should be 'e', 'i' or 'g'");
+              "first letter after 'rv32' should be 'e', 'i' or 'g'");
   }
+
+  EXPECT_EQ(toString(RISCVISAInfo::parseArchString("rv64k", true).takeError()),
+            "first letter after 'rv64' should be 'e', 'i' or 'g'");
 }
 
 TEST(ParseArchString, RejectsUnsupportedBaseISA) {
@@ -395,7 +399,7 @@ TEST(ParseArchString, AcceptsAmbiguousFromRelaxExtensions) {
 TEST(ParseArchString, RejectsRelaxExtensionsNotStartWithEorIorG) {
   EXPECT_EQ(
       toString(RISCVISAInfo::parseArchString("rv32zba_im", true).takeError()),
-      "first letter should be 'e', 'i' or 'g'");
+      "first letter after 'rv32' should be 'e', 'i' or 'g'");
 }
 
 TEST(ParseArchString,