[flang] Fix character initialization after continuation

[luporl]

Summary

The insertion of a space on a line continuation right before
a character literal was confusing TokenSequence::ToLowerCase(),
that was unable to identify the character literal as such,
causing it to be converted to lower case.

Fix this by skipping spaces in the beginning and end of each
token, before testing for token type.

Fixes https://github.com/llvm/llvm-project/issues/62039

[klausler]

flang/lib/Parser/token-sequence.cpp

158

Always use braced initialization in the parts of f18 that use modern C++, please, just like the immediately preceding line.

[luporl]

Use braced initialization and rebase to current head.

flang/lib/Parser/token-sequence.cpp

 TokenSequence &TokenSequence::ToLowerCase() {
   std::size_t tokens{start_.size()};
   std::size_t chars{char_.size()};
   std::size_t atToken{0};
   for (std::size_t j{0}; j < chars;) {
     std::size_t nextStart{atToken + 1 < tokens ? start_[++atToken] : chars};
     char *p{&char_[j]};
     char const *limit{char_.data() + nextStart};
+    const char *lastChar = limit - 1;
     j = nextStart;
+    // Skip leading whitespaces
+    while (p < limit - 1 && *p == ' ') {
+      ++p;
+    }
+    // Find last non-whitespace char
+    while (lastChar > p + 1 && *lastChar == ' ') {
+      --lastChar;
+    }
     if (IsDecimalDigit(*p)) {
       while (p < limit && IsDecimalDigit(*p)) {
         ++p;
       }
       if (p >= limit) {
       } else if (*p == 'h' || *p == 'H') {
         // Hollerith
         *p = 'h';
       } else if (*p == '_') {
         // kind-prefixed character literal (e.g., 1_"ABC")
       } else {
         // exponent
         for (; p < limit; ++p) {
           *p = ToLowerCaseLetter(*p);
         }
       }
-    } else if (limit[-1] == '\'' || limit[-1] == '"') {
-      if (*p == limit[-1]) {
+    } else if (*lastChar == '\'' || *lastChar == '"') {
+      if (*p == *lastChar) {
         // Character literal without prefix
-      } else if (p[1] == limit[-1]) {
+      } else if (p[1] == *lastChar) {
         // BOZX-prefixed constant
         for (; p < limit; ++p) {
           *p = ToLowerCaseLetter(*p);
         }
       } else {
         // Literal with kind-param prefix name (e.g., K_"ABC").
-        for (; *p != limit[-1]; ++p) {
+        for (; *p != *lastChar; ++p) {
           *p = ToLowerCaseLetter(*p);
         }
       }
     } else {
       for (; p < limit; ++p) {
         *p = ToLowerCaseLetter(*p);
       }
     }

flang/test/Parser/continuation-before-char.f90

+! RUN: %flang_fc1 -fdebug-unparse %s 2>&1 | FileCheck %s
+! Continuation right before character literal.
+subroutine test()
+! CHECK: CHARACTER(LEN=3_4) :: a = "ABC"
+  character(len=3) :: a =&
+"ABC"
+end subroutine